Diseases occur when people are infected with pathogens, such as bacteria or viruses. A person's immune system is a network of cells and their responses that protect the body from disease. When the immune system detects pathogens, it produces antibodies. These are special proteins that help fight disease and infections.

Some diseases can be passed from person to person. If enough people get sick, it can lead to an outbreak, or a sudden increase in the number of infected people in a place. However, if a particular number of people in a population are immune, or resistant to the disease, then the disease is passed around as easily. As a result, the disease becomes rarer over time and can even be eliminated altogether.

Two Ways To Become Immune

Herd immunity, also known as community immunity or the herd effect, 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 spread of the disease is lessened, making it less prevalent. This protects many people, including those who are more at risk from infection, such as elderly people.

There are two main ways that people can become immune to a disease: through natural exposure or through immunization, or a shot. In both cases, the human immune system recognizes any pathogens that it has been exposed to before. When it recognizes the familiar pathogen, the immune system is able to produce antibodies much more quickly to fight sickness. A new type of vaccine creates immunity by using artifical mRNA (messenger RNA) made from the pathogen to stimulate the immune system to develop antibodies against a pathogen. Messenger RNA reads DNA to create proteins.

The first way that people can become immune to a disease is through natural exposure. This is why an adult who has already had chickenpox (varicella) as a child is unlikely to contract the disease a second time. The person's body can recognize chickenpox and produce antibodies much more quickly the second time around. However, this method of immunity relies on getting sick in the first place. This is risky, because it depends on the fact that the immune system is strong enough to fight the illness and recover.

The second way that people can become immune is through immunization. This is a relatively safe and effective option.

Modern Vaccines Often Use Modified Injections of Pathogens

Vaccination has a long history with accounts of smallpox (variola) innoculation methods being practiced in 16th century China and India. Smallpox was a common disease that killed countless people and left those who survived with disfiguring scars. These early vaccination methods involved rubbing the pus from the scab of someone infected with smallpox onto an uninfected person's arm. This techinque was popularized in the United States by Puritan minister Cotton Mather in a 1721 smallpox outbreak in Boston, Massachusetts. Mather learned the technology from Onesimus, a West African man he kept enslaved. But this technique sometimes killed those it tried to immunize.

In the 18th century, English scientist Edward Jenner developed a safer form of immunization. Instead of innoculating people with a healthy sample of the smallpox virus, Jenner used a related, and less harmful, pathogen. Cowpox was a similar disease to smallpox, but it was not as severe. Jenner noticed that women who milked cows often contracted cowpox, and these women did not seem to get sick during smallpox outbreaks. He had a theory that the two viruses were similar and that exposure to cowpox would make an individual immune against 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, and he did not contract smallpox.

Medical science has come a long way since then. Today's vaccines work by injecting a weak or modified version of a pathogen, or mRNA based on the pathogen, into the body without getting the person sick. When a person receives a vaccine, the body produces an immune response. Now the person's immune system already knows how to make the antibodies that are required to fight the disease. That way, people are immune if they are exposed to the pathogen in the real world.

In certain cases vaccination can produce side effects or allergic reactions, but in general vaccination is a relatively safe and uncomplicated way to generate herd immunity.

Creating Herd Immunity

For herd immunity to take effect, a certain percentage of the population must be immunized. This number is different for every disease. It depends on many factors, including how easily the disease spreads and whom it infects.

Thanks to the effects of herd immunity, vaccines protect more people than just the individuals who receive them. If enough members of the population are immunized, then people who do not receive vaccines are less likely to get sick.

For example, the flu kills nearly 36,000 people per year in the U.S. alone. It can be deadly for people with weak immune systems, such as children and the elderly. Flu shots are effective only if the person has a strong immune response. As a result, the vaccine doesn't always help people who have weak immune systems and need it the most. So, if strong and healthy people get vaccinated, then flu outbreaks can be contained and people at risk can be protected.

It may be impossible to vaccinate every single person on the planet. But if enough people are vaccinated, then herd immunity can lead to the eradication of diseases.