It was just a matter of time before someone discovered that Madagascar is a museum for viruses. The discovery came when a team of American and English scientists perused the genome of the gray mouse lemur. Nestled among its genes were segments of DNA that bore a remarkable resemblance to HIV. How on Earth could a deadly virus’s genes become part of a primate’s own genome? Some kinds of viruses, known as retroviruses, replicate by inserting their DNA into host cells, where their DNA can guide the production of new viruses. But many studies indicate that sometimes these viruses infect the cells that will give rise to sperm and eggs. The virus ends up in a fertilized egg and gets passed down to ever cell in the developing embryo–including its own sex cells. Now the virus gets passed down through the generations. It may still retain the ability to infect other cells for a while, but mutations typically knock out that ability. Instead, the virus can only insert copies of its DNA back into its own host cell’s genome. Over millions of years, this viral DNA spreads through the host genome. Our own DNA contains 98,000 stretches of this virus DNA, plus 150,000 tiny viral fragments, making up about 8% of our genome – about five times more DNA than the DNA that encodes proteins.