Biology Now, 2e

A Critical Choice ■ 33

No one is

vaccinated

Sick people are contagious; disease

spreads through the population.

Some people are

vaccinated

Most people are

vaccinated

Fewer sick and contagious people;

disease spreads, but not as much.

Very few sick, contagious people;

disease does not spread much.

Not vaccinated, healthy

Vaccinated, healthy

Not vaccinated, sick and contagious

Before an outbreak After an outbreak

Figure 2.11

Vaccine prevalence and herd immunity

When the majority of the population is not vaccinated, an outbreak of disease will spread further and

may cause disease and death in members of the population who are too young to be vaccinated or

have a compromised immune system. However, when most of the population is vaccinated against

a contagious disease, the disease will spread little during an outbreak; this tighter containment of a

disease is the result of what is known as herd immunity. Image modified from the National Institute of

Allergy and Infectious Diseases (NIAID).

Q1: What happens to an immunized person when a disease spreads through a population?

(Hint: In the graphic, follow an immunized individual before and after a disease spreads.)

Q2: Explain why a disease is less likely to spread to vulnerable members of a population if most

people are immunized.

Q3: How does vaccination help an individual person? How does it help that person’s community?