Understanding the Different Types of Pathogens

A doctor looking at a petri dish full of pathogens

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Pathogens, or microscopic causes of disease, abound in the world we live in. These nucleic-acid based microbes can come in various forms, from viruses to fungi. However, as varied as they may be in type and structure, pathogens all have one thing in common: In order to cause disease, they generally invade a host. Pathogens are transmitted in various ways, including through the air, sex, blood, and other bodily fluids, or through the fecal-oral route.

Types of Pathogens

Typically, pathogens fall into one of the following four categories. Note that not all viruses, bacteria, fungi, and parasites cause disease, though many do.

  • Viruses: These microscopic infectious agents require a living host to replicate and thrive. Viral pathogens accomplish this by entering the human body and invading a cell where viral replication occurs and then spreads to other cells. Viruses have no cell membrane and no metabolism. Examples of viruses range from mild illnesses like the common cold and stomach flu to human immunodeficiency virus (HIV) and hepatitis C.
  • Bacteria: Many bacteria do not cause disease and therefore are not pathogens, but some are. These microscopic organisms usually appear in shape as rods, spirals, or spheres. Bacteria are usually larger in size than viruses. They have nuclei but do not have a cell membrane surrounding it, unlike a eukaryotic cell. Examples of bacterial infections include strep throat, urinary tract infections, and most pneumonias.

Some viral infections, like influenza or HIV, can cause immune problems and may make people more susceptible to bacterial infections.

  • Fungi: Yeast and mold are types of fungi that can cause disease in humans. Fungi are eukaryotes, meaning their cells contain a nucleus along with other components that are enclosed within membranes. Fungi that cause disease are eukaryotic, but often have cell walls made of chitin. Fungal infections can range from mild (athlete's foot, nail fungus, ringworm) to severe (histoplasmosis, aspergillosis, mucormycosis). These life-threatening fungal infections are much more likely to occur in patients who are already immunocompromised—as a result of cancer, organ transplants, or other severe illnesses—and may require toxic antifungals such as amphotericin.
  • Parasites: Like the other types, these organisms inhabit a host and get energy (food) from that host, often causing illness in the process. Parasites are a group of organisms incorporating protozoa (one-celled) and multicellular organisms (helminths, ectoparasites). Examples of parasites causing human illness are tapeworm (which causes digestive blockage, anemia, or brain and nervous system impairment [neurocysticerosis]), plasmodium (which causes malaria), and ectoparasites living on or in the skin, like scabies and lice. Many ectoparasites are actually visible to the naked eye, meaning they can be seen without use of a microscope, unlike other pathogens.

Defending Against Pathogens

Modern day medicine has many ways of combating pathogens such as vaccines, antibiotics, and fungicides, but the human body is also equipped with many mechanisms to defend against pathogens and the illnesses they cause. For example, the immune system and the different types of cells it produces (leukocytes, neutrophils, and antibodies) are able to fight off pathogens.

In addition, some of the signs of illness, such as sneezing and coughing, are actually the body's attempt to expel pathogens from the body. In fact, fevers, while thought of as a symptom of illness, are actually the body's way of elevating its temperature to a level uninhabitable by some pathogens. It is a reactive defense mechanism that can help kill pathogens and restore health.

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