The virulence indicates the character Pathogène, harmful and violent one of a micro-organism (Bactérie or Virus). The virulence of pathogenic a Létal is easily measurable but that of pathogenic for purposes under-létaux is more complex to evaluate. In ecology, virulence can be defined like the selective loss in value of the host due to the infection.

Evolution of virulence

Just like the Resistance to the antibiotics, virulence is a feature of pathogenic which is subjected to the Natural selection and which evolves/moves. The organizations treated to lower their virulence are known as attenuated . It is the basic principle of the Vaccination.

On the contrary, of the researchers showed that certain medical policies can generate rises of virulence.

The purpose of the theory of the management of the virulence, presented by Paul W. Ewald, is to include/understand how virulence evolves/moves in order to make evolve/move the pathogenic worms of the less harmful forms.

Compromise between virulence and transmission

One of the basic theories is that virulence and the Transmission of pathogenic of a parasite are dependant in a relation of compromise. Pathogenic cannot be transmitted more without harming its host more. Using models of ecology and evolution, one can show that there exists an optimal virulence for the pathogenic one (which corresponds to a balance of Nash in Game theory).