Explain human pathogens and their cycling through the environment.
Pathogens
Many diseases—especially infectious diseases—are caused by pathogens. A pathogen is any bacterium, virus, or other microorganism that can cause disease. An infectious disease is one that can be passed from person to person.
Explore the tabs below to learn more about each type of pathogen:
Viruses are very simple - DNA surrounded by a protein coat.
Viruses are considered non-living because they can only replicate when they are able to insert their genetic material into their host.
Viruses consist of genetic material surrounded by a protein coat.
Because viruses are non-living, they cannot be treated with antibiotics. The only way to combat viruses is for our body’s immune system to react to and develop antibodies from the virus.
There is an upside to relying on your body’s immune system to combat a virus: once you’ve had a specific virus, you are unlikely to get sick from that virus again. This is because your body develops antibodies to the virus, which remember and can fight that virus much quicker. You can, however, get sick again if you are exposed to a different strain of the virus.
Explore the viruses below to learn about their symptoms, treatment, and spread:
🤧 Common Cold (click to reveal) Treatment for the common cold involves rest, hydration, and over-the-counter medication.
What is it?
The common cold is caused by various viruses, with rhinoviruses being the most common culprits.
How does it spread?
It spreads through respiratory droplets when an infected person coughs, sneezes, or talks.
What are the symptoms?
Symptoms include a runny or stuffy nose, sneezing, sore throat, and mild fever.
How do we treat it?
Treatment involves rest, hydration, and over-the-counter remedies for symptom relief.
💉 Influenza(click to reveal) The flu vaccine is the best way to avoid the flu.
What is it?
Influenza is caused by the influenza virus, specifically types A, B, and C.
How does it spread?
Flu viruses spread mainly by tiny droplets produced when infected individuals cough, sneeze, or talk. A person can contract the flu by touching a surface or object contaminated with flu virus and then touching their own mouth, nose, or eyes.
Flu viruses can be detected in most infected individuals one day before symptoms develop and up to five to seven days after becoming sick. People with flu are most contagious during the first 3 days of their illness.
What are the symptoms?
It primarily affects the respiratory system, causing symptoms like fever, fatigue, cough, and body aches. In severe cases, it can lead to pneumonia and other complications.
How do we treat it?
Annual flu vaccines help prevent infection and reduce its severity. Infected individuals are advised to rest, hydrate, and use over-the-counter medications for symptom relief. If caught early enough, antiviral medications can be administered to lessen the severity and length of the illness.
🧴 Chickenpox (click to reveal) Chickenpox most commonly affects unvaccinated children.
What is it?
Chickenpox results from the varicella-zoster virus (VZV).
How does it spread?
Chickenpox is highly contagious and spreads through respiratory droplets or direct contact.
What are the symptoms?
It causes an itchy rash with small, fluid-filled blisters.
How do we treat it?
Vaccination has significantly reduced its prevalence. Infected individuals are advised to rest, hydrate, and utilize over-the-counter medications for symptom relief. Calamine lotion and oatmeal baths can reduce itching.
When I was a child, I had chicken pox and had to take oatmeal baths and my parents put cream all over the blisters. By the time my younger brother and sister were born, the vaccine had been developed and they received the vaccine and did not have to suffer from the illness.
🩹 Warts (click to reveal) Warts are often on fingers, hands, or feet.
What is it?
Common warts are caused by the human papillomavirus (HPV).
How does it spread?
Warts spread through casual skin contact or shared objects.
What are the symptoms?
They appear as small, grainy skin growths, often on fingers or hands.
How do we treat it?
Treatment options include topical medications, freezing, or laser therapy.
🦠 Covid-19 (click to reveal)
COVID-19 can be diagnosed through a nasal swab test.
What is it?
COVID-19 is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). It emerged in late 2019 and became a pandemic in 2020.
How does it spread?
The COVID-19 virus primarily spreads through respiratory secretions and air droplets. Some infected individuals may be asymptomatic (showing no symptoms) or pre-symptomatic (not yet showing symptoms). Both groups can transmit the virus to others, even without obvious symptoms.
What are the symptoms?
Symptoms range from mild (similar to a cold or flu) to severe (pneumonia, acute respiratory distress).
How do we treat it?
Preventive measures include vaccination, hand-washing, mask-wearing, and social distancing. Infected individuals are advised to rest, hydrate, and utilize over-the-counter medications for symptom relief.
🦟 West Nile(click to reveal)
West Nile and Zika are spread through infected mosquitos.
What is it?
West Nile virus (WNV) belongs to the Flavivirus family, which also includes Zika virus, dengue virus, and yellow fever virus.
How does it spread?
West Nile virus is transmitted by infected mosquitoes.
What are the symptoms?
Most people infected remain asymptomatic, but some develop flu-like symptoms or severe neurological complications.
How do we treat it?
Prevention involves mosquito control and protective measures during peak mosquito activity. Infected individuals are advised to rest, hydrate, and utilize over-the-counter medications for symptom relief. There are no specific antiviral medications for treating WNV.
🦠 SARS (click to reveal) There have been no cases of SARS since 2004.
What is it?
Severe acute respiratory syndrome (SARS) is a viral respiratory disease caused by the SARS-CoV-1 virus, which is the first identified strain of the SARS-related coronavirus. It emerged in 2002-2003 and led to a global outbreak. No cases of SARS-CoV-1 have been reported worldwide since 2004.
How does it spread?
The primary route of transmission is respiratory droplets or contact with contaminated surfaces.
What are the symptoms?
Symptoms include high fever, cough, and difficulty breathing.
How do we treat it?
There is no specific antiviral treatment for SARS. Treatment mainly focuses on supportive care to relieve symptoms and prevent complications. Infected individuals are advised to isolate, rest, and hydrate.
🐪 MERS (click to reveal)
Camels are a reservoir for MERS.
What is it?
Middle East Respiratory Syndrome (MERS) is a viral respiratory illness caused by the Middle East Respiratory Syndrome Coronavirus (MERS-CoV).
How does it spread?
Camels are a reservoir for the virus, and human transmission is rare but serious. MERS is primarily transmitted through respiratory droplets.
What are the symptoms?
It causes severe respiratory illness.
How do we treat it?
To reduce the risk of MERS use insect repellent, wear long-sleeved shirts and long pants to prevent mosquito bites, and be cautious when traveling to areas where MERS is prevalent. There is no specific antiviral treatment for SARS. Treatment mainly focuses on supportive care to relieve symptoms and prevent complications. Infected individuals are advised to isolate, rest, and hydrate.
🦟 Zika(click to reveal)
Pregnant women are most at risk from Zika virus due to potential birth defects.
What is it?
Zika virus is transmitted by infected mosquitoes.
How does it spread?
Zika is primarily spread through the bite of infected mosquitoes, particularly the Aedes mosquitoes. It can also spread through sexual contact with an infected person.
What are the symptoms?
Most infections are asymptomatic or cause mild symptoms. Pregnant women are at risk due to potential birth defects (microcephaly) in their babies. Zika is linked to an increased risk of Guillain-Barré syndrome, a rare neurological disorder.
How do we treat it?
To reduce the risk of Zika use insect repellent, wear long-sleeved shirts and long pants to prevent mosquito bites, and be cautious when traveling to areas where Zika is prevalent. There is no specific antiviral treatment for Zika. Infected individuals are advised to rest, hydrate, and utilize over-the-counter medications for symptom relief.
Bacteria
Bacteria do not have a nucleus - their DNA is free-floating in the cell.
Bacteria are living single-called organisms that can break down the tissues of infected organisms for food or release toxins that interfere with the normal activities of the hosts.
Bacteria don’t want to make us sick, but they do want a home that can provide them with a safe place to live and food to eat and our bodies make excellent homes for bacteria.
Bacterial infections can be treated with antibiotics. However, it can take some time for an antibiotic to kill all the bacteria, so if you are given an antibiotic by your doctor, it is important to take the entire course of antibiotic that they prescribe. Otherwise, the antibiotic might only kill the most vulnerable bacteria and leave the more resilient bacteria behind, increasing the prevalence of the more resistant strain.
Explore the bacterial infections below to learn about their symptoms, treatment, and spread:
💋 Botulism (click to reveal) Botulism can spread through contaminated food.
What is it?
Botulism results from exposure to the neurotoxin produced by Clostridium botulinum bacteria.
How does it spread?
Exposure can occur through ingestion of bacterial spores or ingestion of pre-formed toxins in contaminated food. Infants can contract botulism by eating honey.
What are the symptoms?
Symptoms include muscle weakness, paralysis, and breathing problems.
How do we treat it?
Antitoxin is administered to prevent further damage. Depending on the severity, patients may need hospitalization, ventilator support, and/or rehabilitation.
💉 Anthrax(click to reveal) Different exposure routes for anthrax bacteria result in different symptoms.
What is it?
Anthrax results from exposure to the neurotoxin produced by Bacillus anthracis bacteria.
How does it spread?
Exposure can occur through ingestion of bacterial spores or ingestion of pre-formed toxins in contaminated food or undercooked meat. Individuals can also contract anthrax through cuts or sores on the skin. Anthrax spores can also be inhaled or injected.
What are the symptoms?
Absorption route: Raised, itchy bump that develops into a painless sore with a black center, swollen lymph nodes, and fever.
Ingested route: Nausea, vomiting, abdominal pain, headache, loss of appetite, severe bloody diarrhea, and swollen neck.
Inhaled route: Sore throat, mild fever, fatigue, chest discomfort, shortness of breath, coughing up blood, and painful swallowing.
How do we treat it?
Antibiotics inhibit bacterial growth, antitoxin therapy removes toxins caused by infection, and, in severe cases, surgery to remove infected tissues.
🐀 Plague (click to reveal) During the black death plague outbreak, doctors wore a plague mask thought to protect them from infection.
What is it?
Plague results from exposure to the neurotoxin produced by Yersinia pestis bacteria.
How does it spread?
Exposure can occur through flea bites or handling infected animals.
Pneumonic Plague: Breathing difficulties, chest pain, cough, and bloody sputum.
Septicemic Plague: Abdominal pain, diarrhea, bleeding, and shock.
How do we treat it?
Antibiotics inhibit bacterial growth and antitoxin therapy removes toxins caused by infection.
☕️ Strep Throat (click to reveal)
Children are more likely to get strep throat.
What is it?
Strep throat is a bacterial infection caused by Streptococcus pyogenes, also known as group A streptococcus.
How does it spread?
Strep throat is spread through coughing or sneezing and touching contaminated surfaces. Children are most likely to get strep throat in the winter and early spring.
What are the symptoms?
Strep throat causes a sore throat, swollen tonsils, painful swallowing, petechiae on the roof of the mouth, fever, chills, headache, swollen neck lymph nodes, nausea, and/or vomiting.
How do we treat it?
Antibiotics inhibit bacterial growth, over-the-counter pain relievers control symptoms, and rest, hydration, and warm fluids aid in recovery.
🫁 Tuberculosis(click to reveal)
TB can cause severe and long-lasting respiratory symptoms.
What is it?
Tuberculosis (TB) results from exposure to the neurotoxin produced by Mycobacterium tuberculosis bacteria.
How does it spread?
It is spread through coughing, sneezing, or singing in crowded areas. People with weakened immune systems are more likely to get TB.
What are the symptoms?
There are two types of TB infections:
Latent TB: No symptoms; immune system keeps bacteria under control.
Active TB: over several weeks, the following symptoms appear: cough, sometimes with blood or mucus, chest pain, pain with breathing or coughing, fever, chills, night sweats, and/or weight loss, loss of appetite, fatigue.
How do we treat it?
TB is treated with a multidrug regimen for several months. Infection can be prevented with the BCG vaccine.
💧 Cholera (click to reveal) Cholera is spread through contaminated water.
What is it?
Cholera is an acute, diarrheal illness caused by infection of the intestine with the bacterium Vibrio cholerae.
How does it spread?
The bacterium is commonly found in water or food that has been contaminated by feces from an infected person. Inadequate water treatment, poor sanitation, and hygiene contribute to its spread.
What are the symptoms?
Cholera infection can be mild or asymptomatic, but it can also be severe, around 10% of infections develop watery diarrhea, vomiting, leg cramps, dehydration and shock due to rapid loss of fluids. Death can occur if left untreated.
How do we treat it?
Cholera can be treated successfully byreplacing lost fluids and salts. Antibiotics can kill the bacteria and clear the infection.
Fungi
Fungi are multicellular and can cause disease on the surface of the organism. They can also be found in the mouth, throat, fingernails, and toenails.
Fungi can be very dangerous when they are inhaled and spread from the lungs to other organs. This type of infection can become life-threatening if not treated quickly.
Explore the fungal infections below to learn about their symptoms, treatment, and spread:
🍄 Ringworm (click to reveal) Ringworm can spread through contact with infected organisms or surfaces.
What is it?
Ringworm, also known as Tinea corporis, is a common and contagious fungal infection that affects the skin.
How does it spread?
Ringworm fungus can spread through contact with infected animals or humans. Humans can also contract ringworm by contact with contaminated objects or soil.
What are the symptoms?
Symptoms include a ring-like red rash, itchy scalp, patchy hair loss, and slightly raised, expanding ring rashes.
How do we treat it?
Ringworm is treated with topical creams and oral antifungal medications.
👣 Athlete's Foot(click to reveal) Athlete's foot can quickly spread in locker rooms.
What is it?
Athlete’s foot, also known as tinea pedis, is a common fungal skin infection that usually begins between the toes. It commonly occurs in people whose feet become very sweaty while confined within tight-fitting shoes.
How does it spread?
Spreads through direct or indirect contact (touching affected areas or contaminated items) and thrives in warm, moist environments (showers, non-aerated room floors, swimming pools, sweaty sneakers).
What are the symptoms?
Exposure to this fungus can cause scaly, peeling, or cracked skin between the toes, itchiness, inflamed skin, a burning or stinging sensation, and itchy blisters on the feet.
How do we treat it?
Over-the-counter antifungal creams, sprays, or powders and prescription antifungal medications treat this infection.
Parasites
Protists are single-celled eukaryotic organisms.
Parasitic infections can be caused by either Protists, single-celled eukaryotes, or parasitic worms.
Parasites pass from the environment to humans or other organisms through contaminated water or insect bites.
Parasitic worms can enter organisms through the mouth, anus, nose, and skin. These worms primarily live in the intestinal tract, absorbing nutrients from their host.
Explore the parasitic infections below to learn about their symptoms, treatment, and spread:
🦟 African Sleeping Sickness (click to reveal)
What is it?
African trypanosomiasis, also known as sleeping sickness, is a disease caused by the Trypanosoma brucei parasite.
How does it spread?
The parasite is transmitted to humans through bites of infected tsetse flies (found only in sub-Saharan Africa)
What are the symptoms?
Early symptoms are swelling at the bite site. Later stages of infection can cause headaches, weakness, fever, joint pain, itchiness, weight loss, confusion, sleepiness, insomnia, personality changes, seizures, unconsciousness, and coma.
How do we treat it?
Early diagnosis and prompt treatment are crucial. Medications used to treat the infection depend on the type of infection and disease stage. Use insect repellents, wear long-sleeved clothing, and avoid tsetse fly areas.
🦟 Malaria(click to reveal) Malaria can be fatal if left untreated.
What is it?
Malaria is a serious and sometimes fatal disease caused by a parasite from the genus Plasmodium.
How does it spread?
The malaria parasite is transmitted to humans through the bites of infected female Anopheles mosquitoes.
What are the symptoms?
Malaria can cause a slew of symptoms, such as fever, chills, headache, nausea and vomiting, diarrhea, abdominal pain, muscle or joint pain, and fatigue.
How do we treat it?
Antimalarial drugs (e.g., chloroquine, quinine, mefloquine) are used to treat malarial infections.
🪱 Hookworm (click to reveal) Hookworms live in the small intestines absorbing the nutrients from their host's food.
What is it?
Hookworm infection, caused by the nematodes Ancylostoma duodenale and Necator americanus, affects the small intestine and can lead to various symptoms.
How does it spread?
Hookworm is transmitted through contact with soil contaminated with larvae and is more common in tropical and subtropical regions with poor sanitation.
What are the symptoms?
Many infected individuals are asymptomatic. Symptoms can include an itchy rash at the site of entry, abdominal pain, diarrhea, weight loss, and anemia.
How do we treat it?
Anthelminthic medications treat hookworm infections. Some individuals may be prescribed iron supplements for anemia.
As you can see, there are a wide variety of pathogens causing the myriad infections and diseases found in living organisms.
West Nile virus is transmitted to humans via bites from infected mosquitoes.
Severe acute respiratory syndrome (SARS) is a form of pneumonia. It is transferred by inhaling or touching infected fluids.
Middle East Respiratory Syndrome (MERS) is a viral respiratory illness that is transferred from animals to humans.
Zika is a virus caused by bites from infected mosquitoes. It can be transmitted through sexual contact.
Plague is a disease carried by organisms infected with the plague bacteria. It is transferred to humans via the bite of an infected organism or through contact with contaminated fluids or tissues.
Tuberculosis is a bacterial infection that typically attacks the lungs. It is spread by breathing in the bacteria from the bodily fluids of an infected person.
Cholera is a bacterial disease that is contracted from infected water.
Malaria is a parasitic disease caused by bites from infected mosquitoes. It is most often found in sub-Saharan Africa.
Habitats for Pathogens
Data suggests that climate change can exacerbate hundreds of diseases, regardless of mode of transmission.
Pathogens need specific living conditions to stay alive and these conditions usually involve a host, water, and a specific temperature range. In order to survive and reproduce, they must exist within the viable ranges of their limiting factors. However, because many pathogens have short lifespans, they can quickly adapt to changing environmental conditions, especially if they present an opportunity to colonize new hosts!
Pathogens thrive in warm, humid, equatorial-type climate zones. As our planet warms, equatorial-type climate zones are expanding northward and southward. This means that regions that were once subtropical or temperate are now experiencing equatorial-like conditions. With this shift, pathogens and their associated vectors (such as mosquitoes) are also spreading into areas where they haven’t been prevalent before.
Malaria, dengue fever, and Lyme disease are all illnesses that are expanding their range due to our warming climate. This spread of diseases can have serious consequences for human health because populations in these newly affected regions may lack immunity or effective healthcare systems to combat these diseases.
Poverty exacerbates health disparities by limiting access to basic sanitation and clean water, creating opportunities for infectious diseases to thrive. In impoverished regions, proper waste disposal infrastructure is often lacking. Residents may not have access to garbage collection services or designated disposal sites. As a result, waste accumulates in open areas, streets, or makeshift dumps. Piles of garbage attract pests (such as rats and flies) that can carry pathogens. These pests can then spread diseases, like the plague, to humans. Stagnant water (from improper waste disposal) can become a breeding ground for mosquitoes that carry diseases like malaria, dengue, and zika.
Access to clean and safe drinking water is a fundamental human right, but low-income areas often rely on inadequate water sources, such as contaminated wells, rivers, or communal taps. Waterborne diseases like cholera, dysentery, and typhoid thrive in such conditions, leading to widespread infections.
Despite the above, it's important to understand that pathogens can occur in many environments regardless of the appearance of sanitary conditions, so just because water looks clean doesn’t mean that it can’t have some form of life traipsing around in it! This is why hikers and campers use filter straws or boil water before drinking it.
Take a moment to explore the graph below, looking at the death rates for different countries. Do you see a difference between developed and undeveloped or developing countries?
Pathogens adapt to take advantage of new opportunities to infect and spread through human populations.
As equatorial-type climate zones spread north and south into what are currently subtropical and temperate climate zones, pathogens, infectious diseases, and any associated vectors are spreading into these areas where the disease has not previously been known to occur.
Poverty-stricken, low-income areas often lack sanitary waste disposal and have contaminated drinking water supplies, leading to havens and opportunities for the spread of infectious diseases.
Specific pathogens can occur in many environments regardless of the appearance of sanitary conditions.
Controlling Pathogens
Many steps can be taken to try to lower the chances of disease outbreaks. Explore the most common methods used to control pathogens. Most communities and scientists use multiple methods in tandem to best control pathogens.
🦟 Vector Elimination (click to reveal)
Eliminating or reducing vectors helps break the pathogen transmission cycle.
Targeted use of insecticides can reduce vector populations.
Proper sewage treatment prevents pathogens from contaminating water bodies.
Bacteria break down organic matter during secondary treatment.
UV and chlorine disinfection during tertiary treatment kills pathogens.
Sludge treatment ensures that pathogen-containing sludge is disposed of safely.
🚰 Drinking Water Treatment (click to reveal)
Effective water treatment safeguards against waterborne pathogens.
UV and chlorine disinfection kills pathogens.
Filtration removes microbes and other suspended particles.
Of course, drinking water treatment is less available in poorer countries. Explore the graph below to see how GDP is related to deaths from unsafe drinking water. As expected, as GDP increases, deaths decrease:
💉 Vaccines (click to reveal)
Vaccination is a powerful tool to prevent infectious diseases caused by pathogens (e.g., bacteria and viruses). By stimulating the immune system, vaccines create immunity, reducing the risk of infection and transmission.
Vaccines have their limitations, however, as viruses can mutate rapidly under the right conditions. Mutations can become immune to the power of the vaccine unless the vaccine is re-formulated to match the new mutant version of the disease.
Vaccine availability is also limited by access to vaccines and the ability to afford vaccines. Explore the chart below for different regions and see how vaccine coverage changes from country to country:
📈 Monitoring Host Populations (click to reveal)
Scientists also monitor host populations (humans, animals, or insects) for indications of disease.
It’s not realistic to expect that we can eliminate every infectious disease. Scientists have made and will continue to make great strides in this area. For this course, understanding more about the pathogens that cause various diseases and how to deal with them helps us become better prepared to avoid or contend with dangerous disease outbreaks in the future.
You need to know the basics of the following illnesses: West Nile virus, SARS, MERS, zika, plague, tuberculosis, cholera, and malaria.
You should understand HOW climate change can affect the prevalence of pathogens and their vectors.
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ATP: Lesson - Pollution and Human Health (Topic 8.14) 📖 ATP: Study Guide 📚