The study from the Centers for Disease Control and Prevention in Atlanta represents the first look at the prevalence of hepatitis C in the United States.
"This is what we consider a conservative estimate," said Dr. Harold Margolis of the CDC. "This is everyday Mr. and Mrs. American who live in a household. This doesn't include the homeless and the prison population. The number could be higher."
For reasons that aren't entirely clear, an estimated 1.2 million other people who were once infected no longer have any signs of the virus, Margolis said.
Scientists discovered the virus in 1989.
People who inject illegal drugs or engage in unprotected sex account for most hepatitis C carriers, but people who had blood transfusions before mid-1992 also are at risk.
Hepatitis C and alcohol abuse rank as the leading causes of liver disease. The infection can lead to cirrhosis and liver cancer and results in about 1,000 liver transplants annually in the United States.
The standard treatment is the drug interferon, or a combination of interferon and ribavirin. In some patients, drugs can make all signs of the virus disappear, but it's unclear how long the effect lasts.
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