Pathogens are organisms that can cause disease. The different types of pathogens and the severity of the diseases that they cause are very diverse.

In this article, we look at different pathogens, how they affect people, and the diseases they cause. We also explain how pathogens spread and how to reduce the risk of infection.

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Bacteria, viruses, and fungi are all types of pathogens.

A pathogen brings disease to its host. Another name for a pathogen is an infectious agent, as they cause infections. As with any organism, pathogens prioritize survival and reproduction.

The human body’s immune system acts as a defense against pathogens. The body can easily fight off some pathogens, but others are potentially fatal.

There are five main types of pathogens:


Bacteria are microscopic pathogens that reproduce rapidly after entering the body. They can release toxins that damage tissues and cause illness.

Doctors typically prescribe antibiotics to treat bacterial infections, but some bacteria are becoming resistant to these drugs.

Not all bacteria are pathogenic, though. In the body, there are many types of harmless bacteria, and some may even support essential bodily functions.


Smaller than bacteria, a virus invades a host cell. It then replicates, producing hundreds and thousands of new viruses that go on to infect more host cells. Viruses can pass from person to person in various ways, including:

  • via respiratory droplets that travel through the air
  • through contact with the blood of a person with the infection
  • through contact with the bodily fluids of someone with the infection


There are thousands of species of fungi, some of which cause disease in humans. Common fungal skin conditions include athlete’s foot and ringworm. These conditions are contagious and can spread through person-to-person contact.

A study in Trends in Microbiology found that fungal pathogens are evolving a capacity for memory. They can use signals in the body to anticipate imminent threats to their survival, against which they can then prepare themselves.


These single cell organisms cause disease in their host. They infect other organisms to survive and reproduce.

Protist pathogens affect plants and food crops. Foods containing protists can cause dysentery, which is an infection of the intestines that causes diarrhea.

Protist pathogens can also be parasitic and live in other organisms, such as mosquitoes. Protists cause malaria through mosquito bites.

Parasitic worms

Parasitic worms, also known as helminths, are large enough for people to see with the naked eye, and they can live in many areas of the body. Some worms include:

  • Flatworms: These include tapeworms, which reside in the intestines.
  • Thorny-head worms: This type of worm lives in the intestines.
  • Roundworms: These worms can survive in the gastrointestinal tract and lymphatic system.

Pathogens can cause a variety of different diseases, with some being more severe than others. Human bodies are nutrient-rich and can provide a pathogen with an ideal environment in which to grow and multiply.

The severity of infections that pathogens cause will vary. Some infections may be mild, while others can be life threatening. For example, the common cold is a mild viral infection compared with the lethal Ebola virus disease.

Diseases resulting from bacterial pathogens include:

Some scientists believe that viruses are not living organisms. Some reasons for this include:

  • they do not have cells
  • they cannot reproduce without invading a living cell
  • they do not actively respond to changes in their environment

Viral infections include:

About 300 species of fungi are pathogenic to humans. As with bacteria and viruses, they can have a significant effect on human health.

Fungi cause many different types of illness, including:

  • asthma
  • skin and nail infections
  • lung infections, such as pneumonia
  • bloodstream infections
  • meningitis

Protozoa are responsible for most protist diseases. Protozoa are single cell microorganisms that feed on other microorganisms, organic tissues, and debris. Protist diseases include:

  • dysentery
  • malaria
  • African trypanosomiasis, or sleeping sickness

Parasitic worms cause many diseases, which include:

Pathogens can spread in a variety of different ways. For example, direct skin-to-skin contact during sex can lead to sexually transmitted infections (STIs).

Coughing or sneezing can cause pathogens to spread through tiny droplets in the air. These droplets can contain microorganisms, which other people breathe in.

Microorganisms can also travel straight into the gut when a person consumes contaminated food or water.

Bites from infected insects can also spread disease. For example, ticks with a bacterial infection can cause Lyme disease if they bite someone, and mosquitoes with a viral infection can cause Zika virus disease.

Alongside maintaining good general health, a person can take certain steps to reduce their risk of infection from pathogens. These include:

  • Washing and drying the hands regularly: Washing the hands regularly is crucial in helping stop the spread of disease. People should wash them for at least 20 seconds with soap and water or an alcohol-based sanitizer. It is particularly important to do this after doing outside activities, touching animals, going to the toilet, looking after sick people, and sneezing or coughing.
  • Keeping up to date with recommended vaccines: Vaccines against infectious diseases, such as influenza, whooping cough, and measles, are available for babies, children, and adults. Vaccines stimulate the immune system to recognize pathogens, destroy them, and prevent future infections.
  • Keeping surfaces clean: Contaminated surfaces or objects can transfer disease. A study at an airport in Finland found viral contamination at multiple sites on surfaces that people frequently touch.
  • Cleaning kitchens and bathrooms: It is important to keep counters and kitchen surfaces clean, particularly before preparing food. The bathroom can also have a high concentration of infectious agents.
  • Staying home when ill: People with infections should avoid going to school or work or having close contact with others to reduce the chances of spreading a pathogen.
  • Avoiding insect bites: People can do this by wearing insect repellent and sensible clothing, such as long-sleeved shirts, pants, and a hat, in areas where they are likely to encounter insects.
  • Practicing safe sex: Employing safe sex practices, including using condoms and getting regular sexual health checkups, can reduce the chance of STIs.
  • Getting medical advice: A person can talk to a medical professional if there are signs of infection or treatment is not working.

Pathogens affect all living organisms and cause illness to humans in a variety of different ways.

They can damage tissues or cells by producing toxins as they replicate.

While some pathogens cause mild problems, others can be life threatening. Medical experts are continuing to research diseases that result from pathogens to learn more about their causes and how to treat them.