Every day, your body meets millions of tiny germs, such as viruses and bacteria. Most of them are harmless, but some can make you sick. Luckily, your body has its own army: the immune system.
When a dangerous germ enters the body, special white blood cells notice that it does not belong there. They produce antibodies, proteins that attach to the germ and help destroy it. This takes time, often several days, and during those days you may feel ill. But the immune system also does something clever. After the fight, it keeps "memory cells" that remember the germ. If the same germ returns, the body recognizes it and reacts much faster, often before you feel anything at all.
A vaccine uses this memory. It shows the immune system a harmless version of a germ, or just a small part of it, so the body can practice without getting really sick. It is a little like a fire drill at school: nobody is in danger, but everyone learns what to do if a real fire happens.
The idea is more than two hundred years old. In 1796, an English doctor named Edward Jenner tested an old belief: people who had caught cowpox, a mild disease, seemed to be protected from smallpox, a much more dangerous illness. His work led to the first vaccine. In 1980, after many years of vaccination, the world was declared free of smallpox.
After a vaccine, some people have a sore arm or a light fever for a day or two. This is usually a sign that the immune system is learning. Vaccines also protect the people around us. When most people in a community are vaccinated, a germ has a hard time spreading, so babies and people who cannot get certain vaccines are safer too.