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Viruses are very specialized. A human is never going to become infected with the tobacco mosaic virus, that's too big a jump to make. Yet a tobacco plant and a human are at least distantly related, an alien species will be totally unrelated. Viruses can make a from one animal species to another, but it isn't easy. Living in close proximity to animals can give the virus enough opportunities to hit on the correct mutation to infect humans. Think of a virus like a key used to unlock the DNA of the cell. If the virus doesn't have the right key, it can't infect the cell. They key required to infect a pig is a bit different than the key required to infect a human, but a fortuitous (from the viruses perspective, that is) mutation can leave it with the right key to infect a human cell. The key required to infect a cell from a tobacco plant is so different than the key required to infect a human, that it is just not going to be able to mutate enough.
A bacteria just needs someplace warm, wet and with food.
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