Bornaviruses, a type of RNA virus that causes disease in horses and sheep, first inserted their genetic material into ancestral human DNA at least 40 million years ago. The borna virus can infect mammals and birds, but scientists know little about its effects on its victims. In some species it seems to be harmless, but it can drive horses into wild fits. The horses sometimes kill themselves by smashing in their skulls. In other cases, they starve themselves to death. Some scientists have even claimed that borna viruses alter human behavior, playing a role in schizophrenia and bipolar disorder, although others say there is no solid evidence of a link.
Borna viruses are not the only viruses lurking in our genome. Scientists have found about 100,000 elements of human DNA that probably came from viruses. But the borna virus belongs to a kind of virus that has never been found in the human genome before. Its discovery raises the possibility that many more viruses are left to be found.