Molecules produced by bacteria that enable them to invade a host, evade host defences and cause disease.

Virulence factors include adherence factors (enabling a pathogen to attach to host cells), invasion factors (enabling a pathogen to invade host cells), capsules (protecting pathogens from immune recognition), toxins (endotoxins and exotoxins), and siderophores (iron-binding factors that allow some bacteria to compete with the host for iron).

Targeting virulence factors has been suggested as an alternative to antibiotics, although they do not clear infections. There is potential for agents targeting virulence factors to be used in combination with antibiotics, which could help to limit the spread of antibiotic resistance.