TABLE 1.

Definitions and proposed revisions

Term	Definition(s) from the literature	Proposed definition
Pathogen	A microbe capable of causing disease (17, 34)	A microbe capable of causing host damage; the definition can encompass classical pathogens and opportunistic pathogens; host damage can result from either direct microbial action or the host immune response
	A microorganism that can increase in living tissue and produce disease (12)
	Any microorganism whose survival is dependent upon its capacity to replicate and persist on or within another species by actively breaching or destroying a cellular or humoral host barrier that ordinarily restricts or inhibits other microorganisms (10)
	A parasite capable of causing or producing some disturbance in the host (29)
Pathogenicity	The capacity of a microbe to produce disease (27, 32)	The capacity of a microbe to cause damage in a host
Virulence	Degree of pathogenicity (33, 34)	The relative capacity of a microbe to cause damage in a host
	Virulence ∝ 1/resistance (8)
	Strength of the pathogenic activity (12)
	Relative capacity to overcome available defenses (31)
	Disease severity as assessed by reductions in host fitness following infection (24)
	Percent of death per infection (7)
	A synonym for pathogenicity (34)
	Property of invasive power (35)
	Measure of the capacity of a microorganism to infect or damage a host (21)
	Relative capacity to enter and multiply in a given host (29)
Virulence factor (or determinant)	A component of a pathogen that when deleted specifically impairs virulence but not viability (33)	A component of a pathogen that damages the host; can include components essential for viability including modulins (16)
	Microbial products that permit a pathogen to cause disease (27)