Label the flu epidemics. How can you best explain the biennial fluctuation in the number of cases? How can you explain the epidemics?

검증된 단계별 안내

Step 1: Identify the pattern of flu epidemics by observing the bar graph. Notice that the number of flu cases peaks significantly every 10 years (at years 10 and 20), indicating major epidemic events at these points.

Step 2: Label the flu epidemics as occurring at these peak years (year 10 and year 20) where the number of cases is highest, representing epidemic outbreaks.

Step 3: To explain the biennial fluctuation (every two years) in the number of cases, consider the concept of herd immunity and virus mutation. After a major epidemic, many individuals gain immunity, causing a drop in cases the following year.

Step 4: Over the next couple of years, immunity in the population wanes or the virus undergoes antigenic drift (small genetic changes), allowing the virus to infect more people again, leading to smaller outbreaks in the years between major epidemics.

Step 5: The major epidemics every 10 years can be explained by larger antigenic shifts or the emergence of new flu strains to which the population has little or no immunity, causing widespread infection and a large increase in cases.

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주요 개념

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Epidemiology of Influenza

Influenza epidemics occur when a significant portion of the population becomes infected in a short period. These outbreaks are influenced by factors such as virus mutation, population immunity, and environmental conditions. Understanding the timing and scale of epidemics helps in predicting and managing flu seasons.

Antigenic Drift and Shift

Influenza viruses undergo frequent genetic changes through antigenic drift (small mutations) and antigenic shift (major changes). These changes can lead to new virus strains that evade existing immunity, causing periodic epidemics. Antigenic drift explains seasonal flu variations, while antigenic shift can trigger pandemics.

Population Immunity and Biennial Fluctuation

The biennial fluctuation in flu cases is often due to the build-up and waning of herd immunity. After a large epidemic, many individuals gain immunity, reducing cases the following year. As immunity declines or new strains emerge, susceptibility increases, leading to another epidemic every two years.