University of Michigan - Ann Arbor

Pathogenic bacteria are bacteria that can cause infectious disease.

Pathogenic Bacteria

Virulence is the degree to which an organism is pathogenic. Highly virulent pathogens will almost always lead to a disease. Less virulent pathogens may cause an initial infection, but they do not always cause severe illness. Some individuals might even be asymptomatic. Two important indicators of virulence are:
- the median infectious dose (ID50) 
- the median lethal dose (LD50)

Virulence

There are four stages of pathogenesis. The pathogen must be able to gain entry to the host, travel to the location where it can establish an infection, evade or overcome the host’s immune response, and cause disease to the host.
- exposure (contact)
- adhesion (colonization)
- invasion
- infection.

Stages of Pathogenesis

A disease is any condition in which the normal structure or function of the body is damaged or impaired.

Disease

- Opportunistic pathogen
- Primary pathogen

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