Virulence factors are substances or products generated by infectious agents that enhance their ability to cause disease. Generally, virulence factors can be summarized into four primary categories including toxins, adhesion factors, evasive factors, and invasive factors.

Toxins are substances that alter or destroy the normal function of the host or host’s cells.

Adhesion factors. No interaction between microorganisms and humans can progress to infection or disease if the pathogen is unable to attach and colonize the host.

Evasive Factors. A number of factors produced by pathogens enhance virulence by evading various components of the host’s immune system.

Invasive Factors. Invasive factors are products produced by infectious agents that facilitate the penetration of anatomic barriers and host tissue.

Note that there are no direct connections between resistance and virulence. However, it is possible for enhanced virulence traits to be added to resistant strains by linkage with virulence genes on plasmids or other genetic elements.