When you get sick with a disease like chicken pox or measles, you will never get that disease again (unless you only had a very mild case). That is because your body makes something called ANTIBODIES which will fight that virus if you are ever in contact with it again.
Our bodies make antibodies ONLY when it is exposed to the virus first, and each virus has it’s own “shape” so that the antibodies made only work for that particular virus. Think of antibodies as a puzzle piece that fits exactly onto the “shape” of the virus. It recognizes the shape, attaches itself and allows the invading virus or bacteria to be destroyed.
If that’s a fact, why do we still get sick??
When a virus enters our body, our body immediately goes to work, but it takes TIME to make antibodies, and during that time the virus or bacteria can take hold and cause illness or even life-threatening infection.
The great thing about antibodies is that once your body has made them, they STAY IN YOU, ready to fight the next time they see that same virus which means the next time the virus can’t make you sick.
Vaccines prepare your body to fight!
When we give someone a vaccine, we are giving their body a chance to make antibodies AHEAD OF TIME to fight the disease. Vaccines are made of weakened or killed versions of the virus. The shape of the virus remains similar enough that our bodies make antibodies just like it would if it were exposed to a full blown infection. Then our bodies have those antibodies ready to fight. It is why vaccines have to be given BEFORE being exposed to a virus or bacteria that can cause us harm.
In a sense, the vaccine tricks the body into thinking it is under assault, and the immune system makes weapons that will provide a DEFENSE when a real infection becomes a threat.
Why do we need boosters??
Sometimes one dose of a vaccine is enough for your body to make antibodies to protect you, but often more than one dose is needed. Some antibodies protect for a lifetime, but others need boosting. For EXAMPLE, measles antibody lasts a lifetime, but antibody to tetanus can fall below a level that protects you, so booster doses are needed.
Some viruses such as the flu can CHANGE their “shape” enough to make the antibodies you already have ineffective. That’s why influenza (flu) vaccine is needed every year.
Newborns are immune to SOME infections because they’ve received antibodies from their mothers. But that immunity begins to fade in the first months of life. Some infections such as whooping cough and hepatitis, babies do NOT get any protection from their mothers.
For a more detailed explanation of the immune system and vaccines, see: