A carrier may still be contagious and transmit the infection to others, even though she or he feels and looks perfectly healthy. Carriers can be called asymptomatic, that is, without any symptoms of illness, or symptomatic, which means having symptoms of the illness. 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.