Outbreaks of influenza occur every year in the United States and are associated with an estimated 10,000 to 40,000 deaths. Figure 20.2 shows the variations in the percentage of total deaths attributed to pneumonia and influenza. Pandemics occur periodically over the years, marked by rapid spread of influenza viruses around the globe and higher than normal morbidity. ■ pandemics, p. 466

Although other factors are involved in the spread of influenza viruses, major attention has focused on their antigenic changeability. Two types of variation are seen: antigenic drift and antigenic shift (figure 23.22). Antigenic drift is seen in interepi-demic years and consists of minor mutations in the hemagglutinin (H) antigen that make immunity developed during prior years less effective and ensure that enough susceptible people are

23.6 Viral Infections of the Lower Respiratory System 587

available for the virus to survive. This type of change is exemplified by A/Texas/77 (H3N2) and A/Bangkok/79 (H3N2), wherein there have been mutations that have altered the H antigen slightly. (The geographic names are the places where the virus was first isolated, the next two numbers represent the year.) The antibody produced by people who have recovered from A/Texas/77 (H3N2) is only partially effective against the mutant H3 antigen of A/Bangkok/79 (H3N2). Thus, the newer Bangkok strain might be able to spread and cause a minor epidemic in a population previously exposed to the Texas strain.

Antigenic shifts are represented by more dramatic changes; virus strains appear that are markedly different antigenically from the strains previously seen. Most likely they arise as a result of rare events in which two different viruses infect a cell at the same

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