7.3: Types of Pathogens

Below are the categories of pathogens that we currently know about. Some are more well-known than others. It is important to remember that often the symptoms - and even disease processes - that we associate with infection are in fact our body’s own immune response to the pathogen. However, knowing the type of pathogen that causes the disease is helpful at not only preventing its transmission but discovering treatment or prevention methods. For example, antibiotics are effective at treating bacterial upper-respiratory infections, but will not be effective against viral upper-respiratory infections.   

• Viruses: Composed of either RNA or DNA packaged in a protein sheath, viruses invade host cells and cause the host cell’s nucleus to reproduce the virus, often killing the host cell in the process (NHGRI, 2024). As the virus is reproduced within the host organism, it may “shed” the virus by releasing virus-laden mucus through coughing or sneezing. Viral shedding is essentially when the infected person is most infectious to others (ACHI, 2020).   

• Bacteria: These are single-cell organisms that come in a variety of shapes, many of which are symbiotic to humans and animals (ex: gut microbiota). Some cause diseases by reproducing so rapidly that they invade healthy tissues, or by releasing toxins that kill off healthy cells (Drexler, 2010).  

• Fungi: These are multicellular organisms that reproduce by creating spores that can be airborne or waterborne, including molds, yeasts, and mushrooms. Most fungi don’t cause illness. When they do, fungal infections can affect the surface (skin) or internal organs (lungs or blood). Sometimes exposure to fungi can simply cause asthma or allergy symptoms (About Fungal Diseases, 2023). 

• Prions: A relatively newly discovered pathogen, prions are only proteins that are abnormally folded. When this protein comes in contact with other proteins, it can change their structure, causing holes to form in tissue. Creutzfeldt-Jakob disease and “mad cow disease” are thought to be caused by prions.

• Parasites: These are single or multi-celled organisms that invade a host and use it to supply nutrients, often causing damage or disease to the host in the process. Although parasites are present in all countries around the globe, certain rural and impoverished areas have higher rates of parasitic infections called Neglected Tropical Diseases (NTDs - so named for their lack of public health attention) which affect a billion people worldwide. 

  • Worms: (aka helminths). Worms attach themselves to the inner organs of the host that ingests them in order to gather nutrients. They often are ingested through contaminated food or water, and attach to the intestines, although they can spread to various parts of the body in their larval stage.

  • Protozoa: Single-cell organisms, these can live both in a host (parasitically) or outside of a host. Protozoa can be transmitted via vectors (such as a mosquito bite) or fecal-oral contamination of water or food, and cause diseases such as malaria, trichomoniasis, and giardia, among others. Malaria is a protozoa transmitted to humans by mosquito bites - and kills more than 400,000 people annually. 

  • Ectoparasites: Typically including ticks, fleas, lice, and mites, an ectoparasite is any arthropod that burrows into the flesh and stays there for a period of time. They can also be vectors that transmit other pathogens through bites (CDC, 2024d). 

Figure \(\PageIndex{1}\): Medical illustration of drug-resistant, Streptococcus pneumoniae bacteria, presented in the Centers for Disease Control and Prevention (CDC) publication entitled, Antibiotic Resistance Threats in the United States, 2019 (AR Threats Report).. (Copyright; CC0 Meredith Newlove. CDC/ Antibiotic Resistance Coordination and Strategy Unit)