Viruses can kill the organism they invade; eventually disappear (the body kills the virus or there are so few new organisms to infect that the virus tends to disappear; or the virus and its host organisms learn to co-exist. Viruses are able to help spread beneficial bacterial mutations quickly throughout the microbiome and beyond. A 2013 study of the human gut virome tracked the identities, abundance, and mutations of native viruses in one person over 2.5 years. There were 478 relatively abundant viruses, most of which had not been previously identified. A majority of the viruses were bacteriophage, the type that infects bacteria. Eighty percent of the viruses persisted for the entire 2.5 years, but they all mutated: some slowly, some quickly, and some so fast that the virus would be deemed a new species within the 2.5 years. Talk about a miniature micro-star wars . . .
(Enriques, Juan, and Steve Gullans, PhD. Evolving Ourselves. Pg 101-103. NY:Current-Penguin Group, 2015)