University of Michigan - Ann Arbor

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

The ID50 is the number of pathogen cells or virions required to cause active infection in 50% of inoculated animals. 

Median Infectious Dose

The LD50 is the number of pathogenic cells, virions, or amount of toxin required to kill 50% of infected animals. The lower the LD 50, the more virulent the pathogen is.

Median Lethal Dose

Virulence factors are produced by individual pathogens, they determine the extent and severity of disease. Virulence factors are encoded by genes. When the genes encoding virulence factors are inactivated, virulence in the pathogen is diminished.

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