Disease. Worldwide, an Estimated 200 Million People Are Infected with the Virus. Some of Them Will Suffer Cirrhosis, Liver Cancer and Even Death. Celebrities Like Steven Tyler of Aerosmith and “American Idol” Have

disease. Worldwide, an estimated 200 million people are infected with the virus. Some of them will suffer cirrhosis, liver cancer and even death. Celebrities like Steven Tyler of Aerosmith and “American Idol” have spoken publicly of their infections.

But mysteries still shroud the disease. Typically spread through drug injections, blood transfusions and sexual contact, hepatitis C can quietly cause liver damage for 20 years or more before victims become aware that they are ill. “Worldwide, it’s causing devastation,” said Brian Edlin, an epidemiologist at SUNY Downstate Medical Center in Brooklyn.

Its origins are even more puzzling. Hepatitis C is a distinct disease from hepatitis A and B; it belongs to an entirely different virus family that includes diseases like West Nile fever and yellow fever. Scientists have searched for years for related viruses in animals to figure out how it evolved into a human disease.

“Identifying the species reservoir of hepatitis C – one of the most common and deadly of all human viruses – has been something of a holy grail in studies of viral evolution,” said Dr. Eddie Holmes, a virologist at Penn State University.

Now scientists have gotten an important clue, finding a close relative in an unexpected host: dogs.

The discovery, published in Proceedings of the National Academy of Sciences, “represents a major step forward,” said Holmes, who was not involved in the research.

The finding came as a surprise to all the scientists involved. Researchers at Pfizer were investigating virus outbreaks in dogs in shelters across the United States. They swabbed the noses of dogs sick with respiratory diseases and searched for viruses. In some cases they could not isolate a known virus, so they sent samples to the Center for Infection and Immunity at Columbia University, where researchers specialize in finding new viruses.

The Columbia center found that six of nine dogs in one outbreak and three of five in another shared the...