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When a pathogen enters the body it starts replicating itself causing an infection. The body responds by creating the antibodies that can fight against these germs and destroy them. The body retains these antibodies for any future infection.

The point to note is that the body needs to have antibodies for each type of pathogens. If a new type of virus attacks the body, the body starts developing antibodies after the attack. For less severe infections, this process is great as the body's immune system is successful in killing the invading germs. However, in the more severe types of germs, the body may not be able to produce enough antibodies to fight the high rate of growth of the germs. The body does create antibodies to fight with the new virus, but it might be too late by then.

This is where vaccination comes in handy. A vaccine contains a much diminished form of the germ. When this weakened pathogen is injected into the body, it does not have the capacity to replicate itself fast enough to cause an infection or a disease, however, the body is able to produce the antibodies to kill the weakened pathogen. Therefore, when the body is invaded by a natural and more potent form of that pathogen, the body has an upper hand as it already has the antibodies to kill the invading germ.