Scientists have discovered a virus that is a tiny bit human, carrying a gene it stole from our ancestors about 100,000 years ago. And this virus may be using our own gene to infect us. Previous studies have demonstrated that viruses can pick up genes from their hosts. But the new discovery, reported Thursday in the journal Science, may be the first known instance of a virus with a human gene acquired in recent evolutionary history.

"This one hits closer to home," said Nels Elde, an evolutionary biologist at the University of Utah who was not involved in the research. The way that viruses replicate makes them particularly good at thieving DNA. They infect a cell, then co-opt its machinery by injecting their own genes. Sometimes this process goes awry, and a stretch of the host's DNA gets bundled into the new viral genes.

Now scientists have discovered a virus that has picked up a gene from humans. Cedric Feschotte, a biologist at Cornell University, and his colleagues found it as they scanned databases of viral genes. Dr. Feschotte and his colleagues discovered the human gene in molluscum contagiosum virus, a distant relative of smallpox. The human gene in question, BC200, is mysterious. In the brain, it produces an RNA molecule whose purpose scientists have yet to pin down.

The RNA molecule encoded by BC200 has a peculiar structure that can sometimes cause our cells to insert it into a new piece of DNA. However it got there, the BC200 gene has stayed in molluscum contagiosum virus for the past 100,000 years. Dr. Feschotte has not found a virus without BC200, a sign that they are using the human gene to their advantage.