Disease and the descent of man

...nothing provides more convincing evidence for the “theory” of evolution than the viruses contained within our DNA. Until recently, the earliest available information about the history and the course of human diseases, like smallpox and typhus, came from mummies no more than four thousand years old. Evolution cannot be measured in a time span that short. Endogenous retroviruses provide a trail of molecular bread crumbs leading millions of years into the past.

...The only way that humans, in thousands of seemingly random locations, could possess the exact retroviral DNA found in another species is by inheriting it from a common ancestor.

-- from Darwin's Surprise: Why are evolutionary biologists bringing back extinct deadly viruses? by Michael Specter.