Asked by: Richard Powling, Woodbridge
Viruses work by invading the host cell and forcing it to make lots of copies of the virus. This normally kills the cell in the process and is therefore a bad thing. But your immune response to a mild virus might protect you from a more serious infection: cowpox infections immunise you against smallpox, for example.
Alternatively, you could be a carrier for a virus that doesn't hurt you much but is much more serious for a competing species: chimpanzees carry an immunodeficiency virus that can be transmitted to gorillas (for example when fighting), though I don't know of an example in humans.
Finally, there have been some early trials in mice with viruses that appear to target tumour cells and kill them, including the Sindbis virus, which is transmitted by mosquitoes in Egypt.
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