Immunizations
When disease germs enter your body, they start to reproduce. Your immune system recognizes these germs as foreign invaders and responds by making proteins called antibodies. These antibodies’ first job is to help destroy the germs that are making you sick. They can’t act fast enough to prevent you from becoming sick, but by eliminating the attacking germs, antibodies help you to get well.
The antibodies’ second job is to protect you from future infections. They remain in your bloodstream, and if the same germs ever try to infect you again — even after many years — they will come to your defense. Only now that they are experienced at fighting these particular germs, they can destroy them before they have a chance to make you sick.
This is immunity. It is why most people get diseases like measles or chickenpox only once, even though they might be exposed many times during their lifetime.
This is a good system for preventing disease. The only drawback is obvious — you have to get sick before you become immune.
Vaccines offer a solution to this problem. They help you develop immunity and create antibodies without getting sick first.