There are two ways to become immune to an infectious disease: to catch it and fight it off so that the body forms antibodies, and to get a vaccine, which introduces a small amount the pathogen so the body can form antibodies without getting sick. People who are immune to a viral or bacterial infection protect not only themselves but the community at large. Herd immunity prevents the disease from spreading into vulnerable populations, limiting outbreaks.
When a community is not immune to a disease, germs spread very quickly. If enough people contract the infection, it leads to an outbreak that affects large portions of the community. When enough people are immune to a disease, however, either through natural immunity of vaccination, the germs have a difficult time spreading, which slows down transmission. This is herd immunity. If someone does get sick, the chances of passing it on are lower since there are fewer people to contract it.
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