Virulence factors are molecules produced by pathogens (bacteria, viruses, fungi and protozoa) that contribute to the pathogenicity of the organism and enable them to achieve the following:

• colonization of a niche in the host (this includes attachment to cells)
• immunoevasion, evasion of the host's immune response
• immunosuppression, inhibition of the host's immune response
• entry into and exit out of cells (if the pathogen is an intracellular one)
• obtain nutrition from the host