What Are the Five Pathogens?

Pathogens are infectious micro-organisms, germs, or biological agents that cause infectious diseases or illnesses in the host human. The ability of a pathogen to cause disease is called pathogenicity. The degree to which an organism is pathogenic is called virulence. There are five main types of pathogens: virus, bacterium, fungus, protozoa, and helminth.

The severity of the diseases caused by pathogens is varied. Some infections are mild whereas others could be life-threatening. For example, a common cold is a mild viral infection compared with the lethal Ebola virus disease or coronavirus. Coronavirus, Anthrax, Human immunodeficiency virus infection or HIV, Epstein-Barr virus, and Zika virus are examples of some pathogens that cause serious diseases.

Types of pathogens

The different types of pathogens that invade the human body and cause health issues include viruses, bacteria, fungi, protozoa, and helminths (parasitic worms).


They are microscopic infectious agents that are smaller than bacteria. Viruses are made up of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) protected by a coating of protein. Once a virus invades the host cells, it uses the host cell to replicate, producing many new viruses that infect more host cells. This process usually damages or destroys the infected cells.

Pathogenic viruses and the diseases they cause:

Antibiotics are not effective in the treatment of viral infections. Depending on the viral infection, antiviral medications are sometimes prescribed.


Bacteria are single-celled, microscopic pathogens having different shapes, such as rods, spirals, or spheres. Bacteria are usually larger than viruses and have a nucleus containing DNA.

They can live in any environment and reproduce rapidly after entering the host body. They release toxins that damage the tissues causing illness. Some bacteria are not pathogenic and support essential body functions.

Pathogenic bacteria and the diseases they cause:

Antibiotics are prescribed to treat bacterial infections.


Fungi are single or multi-celled organisms that can survive outside a host. Their cell contains a nucleus with cellular components enclosed within a thick cell wall. 

There are millions of fungi species, some of them cause diseases in humans. They absorb food and nutrients produced after the secretion of digestive enzymes into their surroundings.

Pathogenic fungi and the diseases they cause:

Life-threatening fungal infections occur in patients who have weak immunity, such as in cases of cancer, organ transplants, or HIV-infected individuals.


Protozoa are single-celled organisms that can move through water and prey on other microorganisms.

When they enter the human host, they feed off parasitically and multiply in their host causing infections and illnesses.

Pathogenic protozoa and the diseases they cause:


Helminths are larger, multi-celled organisms that are also called parasitic worms. These worms inhabit many areas of the body and get food from that host often causing illness in the process. Some worms include:

  • Roundworms: These worms live in the gastrointestinal tract and lymphatic system.
  • Flatworms: Include tapeworms, which reside in the intestines.
  • Thorny-head worms: Lives in the intestines.

Pathogenic helminths and the diseases they cause:

  • Ascaris lumbricoides (roundworm): Ascariasis.
  • Strongyloides stercoralis (threadworm): Strongyloidiasis.
  • Echinococcus species: Cystic echinococcosis, alveolar echinococcosis.
  • Taenia solium (tapeworm): Gastrointestinal complications, cysticercosis.
  • Trichinella spiralis (trichina worm): Trichinosis.
  • Schistosoma mansoni (flukeworm): Schistosomiasis.

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Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. Garland Science; 2002. Introduction to Pathogens. https://www.ncbi.nlm.nih.gov/books/NBK26917/