Virulence of a biological agent is defind as -
For an infectious agent, infectivity refers to the propoion of exposed persons who become infected. Pathogenicity refers to the propoion of infected individuals who develop clinically apparent disease. Virulence refers to the propoion of clinically apparent cases that are severe or fatal. Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 43
Ability of the agent to invade and multiply in host
Propoion of clinical cases resulting in severe clinical manifestation
Ability of the agent to induce a clinically apparent illness
Propoion of clinical cases in a virgin population
Epidemiology