Figure 14.7 Koch ’ s postulates The suspected agent must be present in every case of the disease. Diseased subjects Agent not typically found in healthy subjects Healthy subjects Petri plate Bacterial colonies The agent must be isolated and grown in pure culture. Streaked plates Injection The cultured agent must cause the disease when it is inoculated into a healthy, susceptible experimental host (animal or plant). The same agent must be reisolated from the diseased experimental host.
Figure 14.10 The stages of infectious diseases Incubation period (no signs or symptoms) Prodromal period (vague, general symptoms) Illness (most severe signs and symptoms) Decline (declining signs and symptoms) Convalescence (no signs or symptoms) Time Number of microorganisms or intensity of signs or symptoms
Figure 14.17 Epidemics may have fewer cases than nonepidemics Expected number of cases Time Actual number of cases Disease A Actual number of cases Disease B Epidemic 1 Epidemic 2 Number of cases per 100,000 individuals
Figure 14.18 A page from the CDC ’ s Morbidity and Mortality Weekly Report (MMWR)
Figure 14.20 The interplay of factors that result in nosocomial infections Presence of microorganisms in hospital environment Immunocompromised patients Transmission of pathogens between staff and patients and among patients Nosocomial infection