Immune system would perfectly remember the composition of a virus or bacteria that attacked the body before. Powerful antibodies would be produced by the body after instantly recognizing these foreign bodies. We will not know that the foreign bodies have entered our body but the immune system will give us sufficient protection from them. His is the reason why the same virus that attacked the body will not normally be able to cause diseases in an individual. One example is the chicken pox virus which normally does not affect a person more than once in his life as the immune system would keep it away.