Virulence factors (preferably known as pathogenicity factors or effectors in plant science) are cellular structures, molecules and regulatory systems that enable microbial pathogens (bacteria, viruses, fungi, and protozoa) to achieve the following:

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

Specific pathogens possess a wide array of virulence factors. Some are chromosomally encoded and intrinsic to the bacteria (e.g. capsules and endotoxin), whereas others are obtained from mobile genetic elements like plasmids and bacteriophages (e.g. some exotoxins). Virulence factors encoded on mobile genetic elements spread through horizontal gene transfer, and can convert harmless bacteria into dangerous pathogens. Bacteria like Escherichia coli O157:H7 gain the majority of their virulence from mobile genetic elements. Gram-negative bacteria secrete a variety of virulence factors at host-pathogen interface, via membrane vesicle trafficking as bacterial outer membrane vesicles for invasion, nutrition and other cell-cell communications. It has been found that many pathogens have converged on similar virulence factors to battle against eukaryotic host defenses. These obtained bacterial virulence factors have two different routes used to help them survive and grow:

• The factors are used to assist and promote colonization of the host. These factors include adhesins, invasins, and antiphagocytic factors.
• The factors, including toxins, hemolysins and proteases, bring damage to the host.