Among clinically apparent cases of an infection, the proportion that are severe or fatal is measured by which term?

1. Three proportions describe an agent, each with a different denominator. 2. Infectivity takes exposed persons as its denominator and counts those who become infected. 3. Pathogenicity takes infected persons as its denominator and counts those who develop apparent disease. 4. Virulence takes clinically apparent cases as its denominator and counts those that are severe or fatal. 5. The question fixes the denominator at clinically apparent cases, so virulence is the answer. 6. Immunity of the host is a property of the person, not a proportion of cases. _Source: CDC Principles of Epidemiology in Public Health Practice, 3rd Edition, section Natural History and Spectrum of Disease_