What Are the Different Types of Pathogens?

Virus hepatitis B
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Pathogens, or disease-causing agents, abound in the world we live in. These microbes can come in various forms. However, as varied as they may be in type and structure, pathogens all have one thing in common: In order to cause disease, they must successfully 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:

Viruses: These microscopic infectious agents require a living host to replicate and thrive. Viruses accomplish this by entering the human body and invading a cell where they copy themselves and then spread to other cells. Examples of viruses range from mild illnesses like the common cold and stomach flu to human immunodeficiency virus (HIV) and hepatitis C.

Bacteria: While many bacteria do not cause disease and therefore are not pathogens, some of these microscopic organisms, which usually appear in shape as rods, spirals, or spheres. Bacteria are usually larger in size than viruses, and people are more susceptible to a bacterial infection after their immune system has already been weakened due to a virus. Examples of bacterial infections include strep throat, meningitis, and food poisoning. 

Fungi: Yeast, mold, and mushrooms are all 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. This means that it is harder to kill them and most medications available are less effective than, for example, antibiotics while also causing more side effects to the person taking them.

Examples of fungal infections include ringworm, histoplasmosis, and vaginal yeast infections. 

Parasites:  These organisms inhabit a host and gets its energy (food) from that host, often causing illness in the process. The three main types of parasites that cause human disease are protozoa, helminths, and ectoparasites. Examples of parasites causing human illness are tapeworm (which causes digestive illness), ticks (which cause Lyme disease), and plasmodium (which causes malaria).

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)  that 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.

 

Source:

Alberts B; Johnson A; Lewis J; et al. (2002). "Introduction to Pathogens". Molecular Biology of the Cell (4th ed.). Garland Science.

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