8.15 Pathogens and Infectious Diseases

9 min readjanuary 18, 2023

Riya Patel

Riya Patel

AP Environmental Science ♻️

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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 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 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, 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 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.


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.
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