Any of various molecules that facilitate invasion of host cells by bacteria, viruses, fungi, or other pathogens or help them to evade host defences. Typically proteins located on the cell surface of the pathogen or secreted by the pathogen allow adherence to or penetration of host cells. Moreover, the polysaccharide capsules of bacteria such as pneumococci prevent phagocytosis by host immune cells. Various bacterial toxins are also virulence factors, as are the pili (see pilus sense 1) via which cell attachment can be accomplished.