Introduction
Pathogens are microorganisms such as viruses, bacteria, and fungi that can cause disease in humans. They are found in many environments, including water, food, soil, and other organisms. Pathogens have evolved a wide variety of mechanisms for infecting humans and other animals, including direct contact, inhalation, and ingestion.
Infectious diseases are illnesses caused by pathogens, which can range from mild to severe and even life-threatening. Some examples of infectious diseases caused by pathogens include the flu, tuberculosis, cholera, and HIV. These diseases can have both genetic and environmental causes, and the two factors often interact.
Historically, infectious diseases have taken a large toll on human health and mortality. Some of the most significant epidemics and pandemics in human history have been caused by pathogens, such as the bubonic plague, influenza, and smallpox. In recent years, new and emerging infectious diseases, such as SARS, MERS and COVID-19, have caused widespread concern and have highlighted the ongoing threat that pathogens pose to human health.
To combat the spread of infectious diseases, it's important to understand the pathways by which pathogens infect and spread through human populations, and to implement measures to prevent and control the spread of these diseases. This includes practices such as good hygiene, vaccination, and providing access to clean water and sanitation. Additionally, rapid identification and response to outbreaks, as well as research on new treatments and vaccines, are crucial in controlling the spread of these diseases.
Plague
Plague is a bacterial infection caused by the bacterium Yersinia pestis. It is primarily transmitted to humans through the bite of infected fleas, which are often found on small mammals such as rats and mice. The disease can also be contracted by handling infected animals or by inhaling respiratory droplets from an infected person or animal.
Plague has a long and devastating history, with several pandemics having occurred throughout history, the most famous being the Black Death in the 14th century which killed millions of people in Europe. Plague has several forms, the most common are bubonic plague, septicemic plague, and pneumonic plague. The bubonic plague is characterized by swollen and painful lymph nodes, or "buboes," as well as fever, chills, and weakness. Septicemic plague occurs when the infection spreads to the bloodstream and can cause organ failure and death. Pneumonic plague is a rare form of the disease that affects the lungs and can be transmitted through the air.
Today, plague is considered a rare disease, but it still occurs in some parts of the world, mainly in rural areas of Africa, Asia, and South America. With prompt diagnosis and treatment with antibiotics, the disease can be effectively controlled and fatalities are rare. In addition, measures such as improving sanitation, controlling rodent populations, and educating the public about the disease, can help prevent outbreaks.
Malaria
Malaria is a parasitic disease caused by the Plasmodium parasite, which is transmitted to humans through the bite of infected mosquitoes, primarily of the genus Anopheles. The disease is most prevalent in tropical and subtropical regions, particularly sub-Saharan Africa, where it is a leading cause of morbidity and mortality.
The symptoms of malaria include fever, chills, headache, muscle pain, and fatigue, which can occur in cycles of every few days to a few weeks. In severe cases, the disease can lead to anemia, organ failure, and death.
Malaria has killed millions of people over the centuries, and it continues to be a significant public health problem, with an estimated 216 million cases and 435,000 deaths worldwide in 2018. The World Health Organization (WHO) estimates that more than 90% of malaria cases and deaths occur in sub-Saharan Africa, and children under 5 years of age are particularly vulnerable.
Efforts to combat malaria have included widespread spraying of insecticides to control mosquito populations, as well as the use of bed nets and indoor residual spraying. There are also several drugs available to treat the disease and prevent deaths, although drug resistance is becoming an increasing problem. Additionally, the development of effective vaccines is ongoing, and several are in the clinical trial phases.
Overall, while malaria is preventable and treatable, there is still a lot of work to be done to eliminate the disease globally, especially in the most affected regions, and to protect vulnerable populations.
Tuberculosis
Tuberculosis, also known as TB, is a bacterial infection caused by Mycobacterium tuberculosis. It primarily affects the lungs and is spread through the air when an infected person coughs or sneezes, releasing the bacteria into the environment. People nearby who inhale the bacteria can become infected.
Symptoms of TB include a persistent cough, chest pain, weakness, fatigue, weight loss, and fever. In some cases, the infection can also affect other parts of the body, such as the kidneys, spine, and brain.
It's worth noting that not all people infected with TB will develop the disease, some people have a latent infection which means the bacteria is present in the body but does not cause any symptoms and cannot be transmitted to others. While people with a latent infection are not sick, their immune system is working to keep the bacteria from becoming active.
TB is a leading cause of death globally, particularly in developing countries, and it's estimated that 1/3 of the world's population is infected with TB, with 9 million people developing the disease and 2 million dying each year.
Fortunately, TB is a treatable and curable disease, with a long-term course of antibiotics. However, the emergence of drug-resistant TB has been a growing concern, making it harder to treat and control. To combat TB, efforts are needed to improve case detection, treatment and prevention, as well as research and development of new drugs and vaccines.
West Nile Virus
West Nile virus is a mosquito-borne virus that primarily infects birds, but can also infect humans and other mammals. The virus was first identified in the West Nile district of Uganda in 1937, and it has since spread to many parts of the world, including North America, Europe, and Africa.
The virus is transmitted to humans through the bite of an infected mosquito, typically of the genus Culex. Most people who are infected with the virus do not develop any symptoms, but in some cases, it can cause a fever, headache, nausea, and muscle weakness. In rare cases, the infection can lead to serious neurological conditions such as meningitis and encephalitis.
The highest numbers of infections and deaths from the virus occurred in 2002 and 2003, but since then the disease has been on a decline in the US due to increased efforts to control the mosquito population and prevent infections. However, there are still occasional outbreaks of the disease in some parts of the country.
Overall, West Nile virus is a serious health concern that requires ongoing surveillance and control efforts to prevent infections and protect public health.
Coronaviruses
Coronaviruses are a large family of viruses that can cause respiratory infections in humans and animals. The viruses are named for the crown-like spikes on their surface.
Severe acute respiratory syndrome (SARS) is a form of pneumonia caused by a type of coronavirus called SARS-CoV. It first emerged in 2002 in southern China and spread to several other countries, causing a global outbreak in 2003. The virus is primarily spread through close contact with an infected person, such as through droplets produced when an infected person coughs or sneezes. The outbreak infected over 8,000 people and resulted in nearly 10% of deaths.
Middle East Respiratory Syndrome (MERS) is another respiratory illness caused by a different type of coronavirus called MERS-CoV. It was first identified in 2012 in Saudi Arabia and has since spread to several other countries in the Middle East and Asia. The virus is primarily spread through close contact with infected dromedary camels, but it can also spread from person-to-person.
Another form of coronavirus, SARS-CoV-2, was first identified in Wuhan, China in 2019. It is the virus that causes the disease COVID-19. The disease has spread rapidly across the globe and has caused a global pandemic. It is primarily spread through respiratory droplets produced when an infected person coughs, sneezes or talks. The virus has affected millions of people worldwide, causing widespread illness and death.
Overall, coronaviruses can cause severe respiratory illnesses, and they can be transmitted through close contact with an infected person or animal. Vaccines and treatments are being developed to combat COVID-19, but more research is needed to understand the virus and how to control it.
Zika
The Zika virus is a mosquito-borne virus primarily transmitted by the Aedes mosquito, which also transmits dengue and chikungunya viruses. It can also be transmitted through sexual contact with an infected person.
Most people infected with the Zika virus experience mild symptoms or no symptoms at all. Symptoms include fever, rash, joint pain, and conjunctivitis (red eyes). However, the greatest concern with Zika virus is its potential to cause serious birth defects, particularly microcephaly, in which babies are born with abnormally small heads and brain damage. This is because the virus can be passed from a pregnant woman to her fetus.
The Zika virus was first identified in Uganda in 1947, but it wasn't until 2007 that it caused its first known outbreak, on the island of Yap in Micronesia. Since then, it has spread to many parts of the world, including Central and South America, Central Africa and Southeast Asia. In 2015, the virus was reported in Brazil and quickly spread throughout the region, causing an outbreak that affected over a million people.
Currently, there is no treatment for Zika virus infection and no vaccine to prevent it. Control efforts focus on reducing the mosquito population, promoting personal protective measures and public health campaigns to raise awareness about the disease.
Water-Borne Diseases
Water-borne diseases are caused by pathogens that are transmitted to humans through contaminated water. These pathogens can include viruses, bacteria, and protozoa, and can lead to a variety of illnesses such as cholera, dysentery, typhoid fever, and diarrhea.
In poverty-stricken and low-income areas, lack of proper sanitation and waste disposal can lead to contaminated water supplies, providing opportunities for the spread of infectious diseases. For example, untreated sewage in streams and rivers can cause dysentery, a bacterial infection of the colon that causes diarrhea and stomach cramps. Cholera, another bacterial disease, is contracted from infected water, and can cause severe diarrhea and dehydration, and can be fatal if left untreated.
Water-borne diseases are a significant problem in many parts of the world, particularly in developing countries. The World Health Organization estimates that 2.1 billion people, nearly one-fourth of the world's population, do not have access to sufficient supplies of safe drinking water, and 2.3 billion people lack access to proper sanitation. This puts them at risk of water-borne diseases and other health problems.
Overall, access to clean water and proper sanitation are crucial for preventing the spread of water-borne diseases and protecting public health. Efforts are ongoing to improve access to clean water and sanitation, and to educate communities on how to prevent the spread of these diseases.
Antibiotic Resistance
Antibiotic resistance is a growing global public health concern, and is caused by the overuse and misuse of antibiotics. Antibiotics are drugs used to treat bacterial infections, but with time, bacteria can evolve and develop resistance to these drugs, making them less effective. This means that infections caused by antibiotic-resistant bacteria can be harder to treat, and can lead to increased illness and death.
Antibiotic resistance can occur when patients do not take the full course of antibiotics as prescribed or when antibiotics are overused in human medicine, agriculture, and aquaculture. This can lead to the survival and spread of drug-resistant bacteria, which can then cause more harm.
Antibiotic resistance is a global problem, and new forms of resistance can easily spread across international boundaries. In the United States, at least 2 million people acquire serious infections with bacteria that are resistant to one or more antibiotics each year, and at least 23,000 people die as a direct result of these antibiotic-resistant infections.
One of the most concerning forms of antibiotic resistance is methicillin-resistant Staphylococcus aureus (MRSA), a type of bacteria that is resistant to many antibiotics. MRSA can cause skin infections in the community, but can also cause life-threatening infections such as bloodstream infections, pneumonia, and surgical site infections in hospitals and other healthcare settings.
Overall, to combat antibiotic resistance, it is important to use antibiotics responsibly and only when they are truly needed, and to continue investing in research and development of new antibiotics and alternative treatments.
Frequently Asked Questions
What are pathogens and how do they spread through human populations?
Pathogens are organisms (bacteria, viruses, parasites, fungi) that cause disease in humans. They spread through populations by several routes: direct contact or bodily fluids (SARS, TB), airborne droplets (TB, SARS), vector-borne bites (malaria, West Nile, Zika via mosquitoes), waterborne/fecal–oral contamination (cholera), and zoonotic spillover from animal reservoirs (MERS, plague). Pathogens adapt (antibiotic resistance, new strains) and can persist in unsanitary environments or reservoirs even when places look clean. Climate-driven range shifts let vectors and diseases move into new temperate areas, and poverty or poor sanitation creates hotspots for waterborne and vector diseases. On the AP exam expect questions about transmission modes, reservoir hosts, vector control, and socioeconomic drivers (CED EIN-3.D keywords). Review Topic 8.15 on Fiveable (study guide: https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm), Unit 8 overview (https://library.fiveable.me/ap-environmental-science/unit-8), and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
Why do pathogens keep adapting and finding new ways to infect people?
Pathogens keep adapting because evolution favors any change that helps them survive, reproduce, and spread. Random mutations or gene swaps can make a bacterium or virus better at entering human cells, resisting drugs (antibiotic resistance), or surviving in new environments. Human actions create opportunities: crowded cities, poor sanitation, and contaminated water let pathogens cycle through people (CED EIN-3.D.1, EIN-3.D.4, EIN-3.D.12). Climate shifts also expand vector ranges (mosquito-borne diseases like malaria, West Nile, Zika) into new areas (CED EIN-3.D.3, vector-borne transmission). Zoonotic spillover (pathogens jumping from animals to people) and reservoir hosts give pathogens new hosts (CED EIN-3.D.10). For the AP exam, link these ideas to examples (tuberculosis, cholera, malaria) and causes like sanitation and climate when explaining how disease spreads. For a concise review, check the Topic 8.15 study guide on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
What's the difference between how plague spreads versus how tuberculosis spreads?
Plague and tuberculosis spread in different ways. Plague is a zoonotic, often vector-borne disease: bacteria live in animal reservoir hosts (like rodents) and are transmitted to people by bites from infected vectors (fleas) or direct contact with contaminated tissues/fluids (CED EIN-3.D.5). Tuberculosis (TB) is an airborne bacterial infection that primarily attacks the lungs and spreads when you inhale respiratory droplets from an infected person (CED EIN-3.D.6). Key differences: plague usually requires a reservoir/vector (zoonotic spillover), so controlling animal hosts and vectors matters; TB is spread human-to-human via inhalation, so ventilation, masks, and treating infected people are critical. Both relate to poverty and sanitation: poor areas increase risk of reservoirs/vectors and crowded conditions that help TB transmit (CED EIN-3.D.4, EIN-3.D.1). For more review, see the Topic 8.15 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
I'm confused about climate change and disease spread - how does warming temperatures make diseases move to new places?
Warming lets diseases move by changing where pathogens and their vectors (like mosquitoes) can survive. Higher temperatures expand equatorial-type climate zones poleward, so vector species (Aedes, Anopheles, Culex) and the parasites/viruses they carry (malaria, Zika, West Nile) can live and reproduce in places that used to be too cold (this is “climate-driven range expansion,” CED EIN-3.D.3). Warmer winters also increase overwinter survival and speed up pathogen life cycles, so transmission seasons get longer. Plus, changing precipitation and extreme-weather events create new breeding habitats (standing water) and stress human infrastructure, increasing exposure in low-income areas (CED EIN-3.D.4). For APES, link this to vector-borne transmission, zoonotic spillover, and how pathogen adaptation exploits new opportunities (CED EIN-3.D.1). For a quick topic review, check the Topic 8.15 study guide on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm). For extra practice, try the unit problems (https://library.fiveable.me/practice/ap-environmental-science).
How can water look clean but still have cholera bacteria in it?
Water can look clear but still have cholera because Vibrio cholerae is microscopic and doesn’t change water’s color, smell, or turbidity. Cholera is waterborne and spreads via the fecal–oral route—contaminated sewage, runoff, or infected carriers can introduce bacteria into drinking sources even when water seems “clean.” V. cholerae can attach to plankton or biofilms, survive in low concentrations, and enter a viable-but-nonculturable state, letting it persist until conditions let it infect people. Poverty and poor sanitation create reservoirs and transmission opportunities (CED EIN-3.D.2, EIN-3.D.12). For AP review, focus on “waterborne transmission,” “reservoir host,” and sanitation links; this topic appears in Unit 8 Pathogens (see the Topic 8.15 study guide: https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm). For extra practice, check Fiveable’s APES practice problems (https://library.fiveable.me/practice/ap-environmental-science).
What's the difference between malaria and Zika virus if they're both spread by mosquitoes?
Short answer: they’re different kinds of pathogens and spread a bit differently. Malaria is caused by a parasitic protozoan (Plasmodium) transmitted when infected Anopheles mosquitoes bite you. It causes cyclical high fevers, anemia, organ damage, and can be deadly; treatment uses antiparasitic drugs. Zika is a virus transmitted mainly by Aedes mosquitoes (and can also be sexually transmitted), usually causes mild fever, rash, or none at all in adults, but is dangerous for fetal development (microcephaly). Important APES links: both are vector-borne (EIN-3.D: vector-borne transmission and range shifts as climates change), but malaria = parasitic disease (EIN-3.D.7) while Zika = viral and sexually transmissible (EIN-3.D.11). Prevention overlaps (mosquito control, bed nets, remove standing water), but treatments differ (antiparasitics vs mostly supportive care) and surveillance/health messaging target different risks. For more review, see the Topic 8.15 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
Why are poor areas more likely to have infectious disease outbreaks?
Poor areas are more likely to have infectious disease outbreaks because poverty creates the environmental and social conditions pathogens need to spread. Lack of sanitation and safe waste disposal and contaminated drinking water provide reservoirs for waterborne and fecal–oral pathogens (e.g., cholera). Crowded housing and limited access to healthcare speed person-to-person spread (e.g., TB) and slow detection and treatment, which lets pathogens circulate longer and develop antibiotic resistance. Poor vector control (stagnant water, unmanaged waste) increases mosquito and rodent populations that transmit malaria, West Nile, plague, and Zika. Climate shifts expanding vector ranges make these communities even more vulnerable. On the AP exam, tie answers to CED keywords like waterborne transmission, vector-borne transmission, and sanitation infrastructure. For a quick review, check the Topic 8.15 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm), the Unit 8 overview (https://library.fiveable.me/ap-environmental-science/unit-8), or practice questions (https://library.fiveable.me/practice/ap-environmental-science).
Can someone explain how SARS and MERS are different even though they're both respiratory diseases?
SARS and MERS are both respiratory diseases caused by coronaviruses, but they differ in origin, transmission, and severity. SARS (SARS-CoV, 2002) spreads mainly by inhaling or touching infected fluids and was more efficient at human-to-human transmission; its reservoir involved intermediate hosts (e.g., civet cats) after a zoonotic spillover. MERS (MERS-CoV, 2012) is a zoonotic virus tied to dromedary camels as the reservoir and transmits less efficiently between humans but has a higher case fatality rate (much deadlier per case). Key CED concepts: reservoir host, zoonotic spillover, and pathogen adaptation (EIN-3.D). For AP exam study, remember SARS is listed as pneumonia spread by inhalation/contact and MERS as an animal-to-human respiratory illness. For a quick review, check the Topic 8.15 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm) and more unit resources (https://library.fiveable.me/ap-environmental-science/unit-8).
How does tuberculosis actually spread through the air when someone coughs?
When someone with pulmonary tuberculosis coughs, they release tiny respiratory particles that contain Mycobacterium tuberculosis from their lungs. Most particles are small (≤5 micrometers) and dry quickly into “droplet nuclei” that stay suspended in air as aerosols. Because these nuclei are so small, they can be inhaled deeply into another person’s lungs where the bacteria can establish infection. Risk is higher with prolonged, close exposure, poor ventilation, and crowding—conditions often found in low-income areas (CED EIN-3.D.4). TB’s ability to survive in airborne droplet nuclei and infect new hosts is why it spreads even when sanitary conditions look okay (EIN-3.D.2, EIN-3.D.6). Good ventilation, masks, rapid diagnosis, and treatment lower airborne transmission. For AP review, this ties to pathogen adaptation and environmental cycling—see the Topic 8.15 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm) and grab more practice questions at (https://library.fiveable.me/practice/ap-environmental-science).
What happens when diseases like West Nile virus move into areas where they've never been before?
When a disease like West Nile moves into a new area, several things happen that let it establish and spread. Vectors (mosquitoes for West Nile) expand their range as climates warm (CED EIN-3.D.3), so infected mosquitoes or reservoir hosts bring the virus into places that didn’t have it before (EIN-3.D.1, EIN-3.D.8). Poor sanitation or standing water in low-income areas creates breeding sites and increases human exposure (EIN-3.D.4). Because pathogens can adapt, local mosquito species may become competent vectors and transmission can become seasonal or year-round. Public-health consequences include more human cases, pressure on health systems, and the need for surveillance, vector control, education, and improved sanitation. On the AP exam this shows up under pathogen cycling, vector-borne transmission, and climate-driven range expansion—good to review the Topic 8.15 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm) and Unit 8 overview (https://library.fiveable.me/ap-environmental-science/unit-8). For extra practice, see Fiveable’s practice problems (https://library.fiveable.me/practice/ap-environmental-science).
I don't understand how plague can be transferred through both bites and touching infected fluids - which way is more common?
Short answer: bites are more common—but it depends on the form of plague. Why: The classic bubonic plague is usually spread when fleas (vectors) bite infected reservoir hosts (like rodents) and then bite humans, so vector-borne transmission (bites) is the most common route historically and in many outbreaks. Direct contact with contaminated fluids or tissues (or inhaling respiratory droplets) can transmit plague too—that’s more typical for pneumonic plague, which is less common but much more contagious and dangerous. Remember this fits the CED point that plague can be transferred by infected organisms’ bites or by contact with contaminated fluids/tissues (EIN-3.D.5). For AP review, focus on vector-borne vs. direct/respiratory transmission and reservoir hosts (rodents/fleas). See the Topic 8.15 study guide for more (https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm). For extra practice, try the unit problems (https://library.fiveable.me/practice/ap-environmental-science).
Why is malaria mostly found in sub-Saharan Africa but not other hot places?
Malaria’s distribution comes from a mix of biology, climate, and human factors—not just “hotness.” The Plasmodium parasite needs specific mosquito vectors (mainly Anopheles species) that thrive in warm, humid places and can live long enough for the parasite to complete development inside the mosquito (the extrinsic incubation period). Sub-Saharan Africa has highly efficient vectors (e.g., Anopheles gambiae), lots of P. falciparum (the most severe species), and climate conditions that speed parasite development—so transmission is intense. Social factors matter too: poverty, limited access to healthcare, poor housing and standing-water control, and weaker surveillance let infections spread and persist. Other warm regions may lack the same vector species, have cooler nights that slow parasite development, better mosquito control, or stronger health systems, so malaria isn’t as common. For AP review, this ties to vector-borne transmission and how pathogens cycle through environments (see the Topic 8.15 study guide) (https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm). For extra practice, try the APES problems at (https://library.fiveable.me/practice/ap-environmental-science).
How do you know if water is contaminated with cholera bacteria?
You can’t tell by sight or smell—cholera (Vibrio cholerae) is a waterborne bacterial pathogen that often occurs even when water looks clean (CED EIN-3.D.2). To know for sure, labs test water for V. cholerae or its toxin: common methods are bacterial culture (grows the organism from a sample), PCR (detects V. cholerae DNA), and rapid immunoassay dipsticks that detect cholera toxin or O1/O139 antigens. Public-health teams also test indicator organisms (like E. coli) to detect fecal contamination, since cholera spreads by the fecal–oral route (keyword: waterborne transmission). If you’re studying for APES, remember the curriculum point that poverty and poor sanitation increase contaminated drinking water risk (CED EIN-3.D.4). For more review on pathogens and cholera, see the Topic 8.15 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm) and Unit 8 overview (https://library.fiveable.me/ap-environmental-science/unit-8). For practice Qs, check Fiveable’s practice page (https://library.fiveable.me/practice/ap-environmental-science).
What's the connection between sanitary waste disposal and infectious disease spread?
Poor or absent sanitary waste disposal directly increases infectious disease spread by creating pathways and reservoirs for pathogens. When human feces or sewage contaminate drinking water or food (the fecal–oral route), waterborne diseases like cholera spread quickly. Open waste and standing sewage also create breeding sites or attract reservoir hosts and vectors (e.g., mosquitoes, rodents), which helps vector-borne and zoonotic diseases expand. Poverty-linked lack of sanitation therefore raises local pathogen abundance and transmission opportunities, and climate-driven range shifts can move those vectors into new areas (CED: EIN-3.D, EIN-3.D.4, EIN-3.D.12). Effective sanitation—sewers, treated wastewater, sealed latrines, and proper landfill liners—reduces contamination of water supplies and limits vector habitats, cutting disease incidence. This concept appears on the AP exam under Topic 8.15; review the study guide for this topic (https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
Can Zika virus really be transmitted through sexual contact or is mosquito bites the main way?
Short answer: Mosquito bites are the main way Zika spreads, but sexual transmission is real and documented. Details: Zika is primarily a vector-borne virus transmitted by Aedes mosquitoes (bite of an infected mosquito). However, infected people can also pass Zika to sexual partners (through semen and possibly other bodily fluids); sexual transmission is much less common than mosquito transmission but important because it can spread the virus where the mosquito vector isn’t present and poses extra risk for pregnant people (congenital Zika syndrome). This fits EIN-3.D: vectors and pathogen adaptation can expand disease spread. On the APES exam, Zika is a good example of vector-borne disease that also has an alternate human-to-human route (CED: EIN-3.D.11). For a quick review, see the Topic 8.15 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/pathogens-infectious-diseases/study-guide/xwGfaDy8boZiAkBqBrzm) or the Unit 8 overview (https://library.fiveable.me/ap-environmental-science/unit-8). Practice AP-style questions are at (https://library.fiveable.me/practice/ap-environmental-science).