Virulence of a biological agent is defind as -: (A) Ability of the agent to invade and multiply in host, (B) Propoion of clinical cases resulting in severe clinical manifestation, (C) Ability of the agent to induce a clinically apparent illness, (D) Propoion of clinical cases in a virgin population
Answer is B. 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