Koalas are having a very hard time. At least 100 years ago they picked up a retrovirus, much like the Feline Leukemia retrovirus that has been attacking domestic cats, and it has spread throughout the population. When the diseases get this contagious and prevalent they can insert themselves into the genetic code of their host.
PastPandemics Are in Our Genes
Carl Zimmer, 5 December 2012 (hat tip: The Browser)
To understand what it means to be human, you have to understand koalas. Or, to be more precise, you have to understand how they are dying from a bizarre viral outbreak that has been raging for the past 150 years or so. The koalas are now going through something our ancestors experienced 31 times over the past 60 million years…
Eventually, these outbreaks ended, and the viruses became trapped in their hosts. But they didn’t lose all their viral powers. They could still parasitize their host’s genome. Sometimes a cell would make an extra copy of the viral genes and then insert them back elsewhere in its genome. As a result, our 31 viral invasions gave rise to 100,000 separate chunks of virus DNA. Altogether, they make up at least 8 percent of the human genome.
One specific example of note is that human European population sometime in the distant past was infected by an Aids-like entity. There are some remnants of this in some European’s genetic code, and it has been theorized that this remnant gives Europeans a slightly better possibility of immunity to the disease.