They rewire the immune system, so that what used to make us sick now keeps us healthy.
About 8 percent of our DNA is made up of viruses known as Endogenous Retroviruses (ERVs). Over millions of years, these viruses have embedded themselves in our genome and now play an integral role in the functioning of our immune system. In this short video, The Atlantic’s science writer Ed Yong explains how the very things that once made us sick now keeps us healthy.
Source: The Irony of Viruses