Pathogen-produced molecules that contribute especially to disease as effected by that pathogen.

Virulence factors consist either of proteins or instead molecules that are products of proteins. The underlying can be described as .

Generally a virulence factor does not contribute solely to pathogen's metabolism as unrelated to disease. Instead, an organism should be capable of replicating with some approximation of normal, with solely some degree of loss of virulence absent of the virulence factor.

Examples of virulence factors include exotoxins, , capsules, M protein, IgA protease, siderophores, superantigens, etc. Often underlying virulence factor genes are encoded by such as prophages, plasmids, and .

Identification of a virulence factor involves some means of removal of the virulence factor, either or , with associated loss of virulence. Replacing the virulence factor, either directly as a molecule or instead as expressed from a vector, should then restore virulence. This process can be referred to as .