2. Bacterial Pathogenesis

What are pathogens?

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.

Different types of pathogens

EM image of feline calicivirus which is one type of pathogen
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 MicrobiologyTrusted Source 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.

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