Читать книгу Principles of Virology, Volume 2 - Jane Flint, S. Jane Flint - Страница 34

Determining the number of infected individuals in a population is a primary goal of epidemiological studies. This information is required to establish both the incidence and the prevalence of infection. Incidence is defined as the number of new cases within a population in a specified period. Some epidemiologists use this term to determine the number of new cases in a community during a particular period, while others use incidence to indicate the number of new disease cases per unit of population per period. For example, the incidence of influenza can be stated as the number of reported cases in New York City per year or the number of new cases/1,000 people/year. Disease prevalence, on the other hand, is a measure of the number of infected individuals at one moment in time divided by an appropriate measure of the population. A highly infectious and lethal disease (such as the 1793 epidemic of yellow fever in Philadelphia) may have a high incidence but a low prevalence, because many of the infected individuals either died or cleared the infection. In contrast, a virus that can persist in a host for decades is likely to have high prevalence. An example of high prevalence is provided by hepatitis B virus; of the 300 million to 400 million people infected globally, one-third live in China, with 130 million carriers. For this reason, incidence is an in formative measure for acute or highly lethal infections, whereas prevalence is often used to describe long-lasting or persistent infections.