For example, viruses are very small and do not carry with them all of the materials necessary to reproduce themselves. So, when a virus enters a human body, it needs to find a human cell that it can infect. Then the virus needs to take over some of the machinery of the cell to replicate itself and spread viruses to other cells. Every disease-causing virus that you have heard of has mastered taking over human cells in this way. The method by which the immune system normally handles a virus infection is to kill the infected cell before the virus can reproduce itself. It is a race in which the virus often has an early advantage, because it takes some time for the immune system to detect the virus and then to get enough immune cells involved to be able to kill all the cells with viruses inside of them. At the same time, it is very important that this potent cell-destruction mechanism does not get out of control and start killing lots of cells that do not have viruses in them.

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