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This can sometimes create a happy outcome. For example, the soil-dwelling bacterium Bacillus subtilis has viral genes that help protect it from heavy metals and other harmful substances in the soil.
Other times, viruses can wreak havoc when they bring in new genes. For example, Vibrio cholerae, the bacterium that causes cholera, is harmless in itself. The disease-causing toxin that causes illness is actually made by a virus that at some point smuggled itself into its host’s genome.
Viruses can also influence host genes by where they insert themselves into their host’s DNA. Recent decoding of the human genome shows that viral DNA sequences have been reproducing jointly with our genes for ages.
Some of these DNA sequences stay put, but others seem to move about our genome, jumping from place to place on a chromosome or from chromosome to chromosome. These “mobile elements” take up nearly half of the human genome.
Hemophilia and muscular dystrophy are two human diseases that researchers now believe resulted from mobile elements that, while skipping about the genome, ungraciously barged right into the middle of key human genes.