The same kind of infection can affect people in different ways. Some people become sick only very briefly, then get better, because their immune systems fight off the infection well. Some people become very ill after infection with a virus. Some persons die from infection. After a virus enters the body, the immune system produces substances called antibodies .
Antibodies are like soldiers guarding a castle.
As soon as they see that the virus does not belong in the body, they chase after it and fight it off. Antibodies and also lymphocytes, a type of white blood cell, are always on guard circulating in the blood stream to protect the body against infection. Another way for the body to get antibodies is through a vaccine. Scientists have created substances which make the body produce antibodies to protect against a particular pathogen. Sometimes a booster shot has to be given several years later, because the antibody wears off and has to be reminded to be on guard against infection.