The chain of infection

What is a pathogen?

The first link in the chain of infection is the pathogen.  A pathogen is anything that causes a disease.  Pathogens include:


  • Bacterium A group of microscopic organisms that are capable of reproducing on their own, causing human disease by direct invasion of body tissues. Bacteria often produce toxins that poison the cells they have invaded. Numerous bacteria also live in harmony with the body and are necessary for human existence, such as bacteria that aid in digestion in the gut. Important bacterial diseases include "strep" tonsillitis, pneumonia, and meningitis. (example: bacterial meningitis or strep throat)

  • Virus A term for a group of microbes that are incapable of reproducing on their own, and must invade a host cell in order to use its genetic machinery for reproduction. Viruses are smaller than bacteria, and are responsible for the most common human diseases, the common cold and the "flu" (influenza). Viruses are also responsible for more serious diseases such as AIDS, hepatitis B, and hepatitis C. (example: hepatitis A, hepatitis B, and hepatitis C)

  • Fungus (example: athlete’s foot)



We come in contact with pathogens everyday.  Most of the time our body’s immune system destroys them before they can cause harm.


  • We are considered exposed when we have been in contact with a pathogen.  

  • We are considered infected when a pathogen has entered the body and resulted in disease.  



Whether an exposure results in infection depends on three factors:


  • Dose -  the amount of organisms that enter your body.

  • Virulence - the strength of the organism.  

  • Host resistance - the ability of your immune system to fight infection.


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