Sixty million years ago, a lemurlike animal—an early ancestor of humans and monkeys—contracted an infection. It may not have made the lemur ill, but the retrovirus spread into the animal’s testes (or perhaps its ovaries), and once there, it struck the jackpot: It slipped inside one of the rare germ line cells that produce sperm and eggs. When the lemur reproduced, that retrovirus rode into the next generation aboard the lucky sperm and then moved on from generation to generation, nestled in the DNA. “It’s a rare, random event,” says Robert Belshaw, an evolutionary biologist at the University of Oxford in England. “Over the last 100 million years, there have been only maybe 50 times when a retrovirus has gotten into our genome and proliferated.”
But such genetic intrusions stick around a very long time, so humans are chockablock full of these embedded, or endogenous, retroviruses. Our DNA carries dozens of copies of Perron’s virus, now called human endogenous retrovirus W, or HERV-W, at specific addresses on chromosomes 6 and 7.
This virus was long thought to be "junk DNA", which makes up a fair amount of our genetic material, but doesn't affect us. The new line of research says that this virus, if it is activated at a certain age under the right conditions, may cause changes to human immune systems that lead to the development of not only schizophrenia, but multiple sclerosis and possibly other diseases. The story of how this discovery came about is a fascinating read at Discover magazine. Link
(Image credit: Flickr user ynse)