Virulence factors are factors that are produced by some microorganisms and may cause disease in the host (e.g., toxins, surface receptors,…). Many virulence factors are produced only by specific virulent strains of a bacterium (e.g., enterotoxins for some E. coli strains). The clinical course of a disease usually depends on the interaction of virulence factors and the response of the host. When the balance between virulence factors and host response is favorable to the first one, the infection begins.

Virulence factors in bacteria may be encoded on both chromosomal and bacteriophage DNA, plasmids or transposons. Other virulence factors are acquired by bacteria after infection by a bacteriophage which integrates its genome into the bacterial chromosome. Other virulence factors are encoded on the bacterial chromosome (e.g., cholera toxin).

Virulence factors help bacteria to (1) invade the host, (2) cause disease, and (3) evade host defenses. There are many types of virulence factors such as adherence Factors, invasion Factors, capsules, endotoxins, exotoxins and siderophores.