Pathogens are infectious agents that cause disease. Examples of pathogens are bacteria and viruses. A person's immune system can recognize pathogens. The immune system is a network of cells and their responses that protect the body from disease. When the immune system detects a pathogen, it produces antibodies. These are special proteins that help fight disease and infections.

Some diseases can be passed from person to person. If many people get sick with a particular disease, then an outbreak can happen. An outbreak is a sudden increase in the number of infected people in a certain place. However, if some people in a population are immune, or resistant to the disease, then the disease cannot be passed around easily. Over time, the disease becomes rarer and can even disappear.

Less Likely to Contract Disease a Second Time

Herd immunity is also known as community immunity or the herd effect. It is the way in which an entire community can be protected from an illness or disease by having a certain percentage of individuals immune to that disease. If this happens, then the disease is no longer able to spread. This protects many people, including those who are more at risk from infection, such as elderly people.

The human immune system can recognize pathogens it has been exposed to before. When it sees a familiar pathogen, it can make antibodies more quickly. It can fight sickness better than it did the first time.

People can become immune to a disease through natural exposure. Adults who have already had chickenpox as children might not get the disease a second time. Their bodies can recognize chickenpox. Their immune systems produce antibodies much more quickly the second time. This form of immunity is risky because it relies on people getting sick in the first place.

Another way that people can become immune is through vaccination, or a shot. This is a relatively safe way to make people immune.

Vaccines Produce Antibodies

Vaccination has been practiced since at least the 1500s. Smallpox (variola) was once a common disease that killed many people. In China and India, pus from the scab of someone infected with the deadly smallpox virus would be rubbed into an uninfected person's arm. This would help the uninfected person better fight off the disease. But the smallpox virus would sometimes kill them.

The same method of fighting smallpox was made popular in the United States by Puritan minister Cotton Mather in 1721 in Boston, Massachusetts. Mather learned of it from Onesimus, a West African man he kept enslaved.

In the 1700s, scientist Edward Jenner made a safer form of vaccination. Cowpox was a similar disease to smallpox. Jenner noticed that people who had contracted cowpox did not get sick during smallpox outbreaks. He guessed that catching cowpox would make a person immune to smallpox.

Jenner decided to test his theory by infecting a young boy with cowpox. The boy fell ill but soon recovered. Jenner then purposely infected the boy with the deadly smallpox virus. Jenner's theory was correct: The boy's exposure to cowpox had made him immune. The boy did not contract the disease.

Today's vaccines inject a weak version of a pathogen into the body or uses human-made mRNA (messenger RNA) based on the pathogen. Messenger RNA reads DNA to create proteins. After receiving a vaccine, the body produces an immune response, but the vaccine does not cause sickness. When people receive a vaccine, their immune systems then know how to produce antibodies. This makes people mostly immune to that disease. They are less likely get sick, even if they are exposed to the pathogen in the real world.

Creating Herd Immunity

Vaccination is a relatively safe way to generate herd immunity. For herd immunity to take effect, a certain number of people must be immunized. This number is different for every disease. It depends on many factors, like how easily the disease spreads and whom it infects.

Vaccines protect more people than just the people who receive them. A certain number of people in the population must be immunized. That way, people who cannot receive vaccines will be less likely to get sick.

For example, the flu kills 36,000 people every year in the United States. It can be deadly for people with weak or developing immune systems, like that of the elderly and children. Flu shots are most effective when they produce a strong immune response. So, the vaccine sometimes doesn't help those who need it most. If strong and healthy people get vaccinated, flu outbreaks can be contained. People who are at risk can be protected.

It may not be possible to vaccinate every single person. But if enough people are vaccinated, herd immunity can lead to fewer disease outbreaks. It can even lead to eradication.