A vaccination is a treatment which makes the body stronger against an infection.

The body fights infections using the immune system, which is made up of millions upon millions of cells including T cells and B cells. An important part of the adaptive immune system is that it is much stronger when fighting a disease that it has already fought against before. Vaccination involves showing the immune system something which looks very similar to a particular virus or bacteria, which helps the immune system be stronger when it is fighting against the real infection.

Vaccination versus Immunization
Another word used for vaccines is immunization. These words mean things that are a little different. Vaccination is when a person is given something to make the immune system learn to fight an infectious disease.

Immunization is when a person's immune system learns to fight an infection. Immunization can happen from vaccination. But immunization can also happen from getting the infection. For example, a person can be immune to hepatitis B if he gets sick with hepatitis B. After a person gets hepatitis B and then gets well, he is immunized from getting it again. A person can also be immunized from hepatitis B by taking the hepatitis B vaccination.

So vaccination and immunization have meanings that are a little different. But when people say these words, they usually mean the same thing. People say immunization to mean the same thing as vaccination.

Herd Immunity 

Herd immunity is an important part of how vaccines work. A herd is a group of animals. Herd immunity happens when most of the animals in a group are immune to an infection. If most animals are immune they cannot get the disease. If they do not get the disease, they cannot give it to other animals. So even one animal who is not immune is safer. If none of the other animals in a herd get the infection, they cannot give the infection to the one who is non immune.

This is important in people too. If 95% of people in a place are immune to a disease, the other 5% are safer. They are not near infected people, so they won't get infected.

The people who are in the 5% are there for many reasons. Some got the vaccine but did not react to it. Their immune system did not learn how to fight it well. Some of them are too sick to get the vaccine. It can be children who are too sick with other diseases to get vaccines. It can be a pregnant woman who cannot get the vaccine because it could hurt her baby. It can be a person with cancer who does not have a strong immune system. It can be an older person who has a weak immune system.

So if everyone in a place gets vaccinated, it protects these people too. If they are not protected by herd immunity, they can get more sick from an infection. They get the infection more easily and they get sicker from it. So it is important that people who are healthy get their vaccinations. It protects the healthy people. But it also is important to protect other people who are old, weak, or sick.

Types of vaccines 

There are different types of vaccines:
 Inactivated vaccines contain particles (usually viruses). These have been grown for the purpose. They have been killed, using formaldehyde or by other means. But the virus still looks intact; the immune system can develop antibodies against it.
 Attenuated vaccines contain live viruses, that have been weakened. They will reproduce, but very slowly, making it an "easy win" for the immune system. Such vaccines cannot be used on patients with a severely weakened immune system, such as those with AIDS, as they are unable to defeat even this very weak virus. 
 Subunit vaccines show antigens to the immune system, without introducing virus material.

There are various other methods. All aim to develop the immune response without allowing the full viral infection to happen.

Safety of vaccination 
Today in modern countries almost all people are vaccinated, which has caused many serious diseases to become rare. However, some people argue against vaccination, as they are worried about possible side effects from the vaccination.

Vaccinations do have some side effects. These include swelling and redness around the injection site, a sore arm, or fever. These effects are because of the immune system fighting with the viruses or bacteria which have been injected. Very rarely, the immune system overreacts so much to the virus that the immune system damages other areas in the body.

As well as these real side effects of vaccinations, some people believe that vaccines cause other serious problems like autism, brain damage, or diabetes. There is no evidence for this. Almost all doctors and scientists believe that vaccination does not cause any of these things.

Overall, the vast majority of medical professionals and scientists believe that vaccinations are a good thing and that the benefits of avoiding diseases are far greater than the very small risk of side effects. All medical organizations around the world, including the World Health Organization(WHO), the American Medical Association, the American Academy of Pediatrics, and the United States Centers for Disease Control support vaccination.

Origin
The word "vaccine" was created by Edward Jenner. The word comes from the Latin word vacca, meaning cow. A virus that mainly affects cows (Cowpox) was used in the first scientific demonstration that giving a person one virus could protect against a related and more dangerous one.

History of vaccination 

The first vaccination ever was for smallpox. In 1796 an English doctor, Edward Jenner, noticed something. He saw that people who got cowpox did not get sick from smallpox. He gave a young boy the cowpox virus to protect him from smallpox. This was done by scratching liquid from cowpox sores into the boy's skin. This same method using the liquid from sores was also used to give people smallpox. People did this so they might get smallpox on one place on their bodies. Then they could pick which body part got scars from smallpox. But sometimes people who did this got very sick from smallpox. Some even died. This was a dangerous thing to do. But people did it because it was less dangerous than getting smallpox.

Edward Jenner gave the boy cowpox in the same way people tried to give smallpox. Six weeks later, he scratched smallpox into the boy's skin. The boy did not get sick from smallpox. This boy was the first person ever to get a vaccination.

It was not until almost 100 years after the smallpox vaccination that the next vaccine for cholera was found in 1879. After that, vaccines for 28 different types of diseases have been found.

References

Other websites 
 World Health Organization about vaccines 
 Sabin Vaccine Institute
 Jenner Institute for Vaccine Research 
 Walter Reed National Vaccine Healthcare Centers Network (US) 
 American Academy of Pediatrics Vaccine Information 
 Vaccination -Citizendium 

Immunization