By Dr. Brian McDonough, Medical Editor

PHILADELPHIA (CBS) - We have all heard of HIV and AIDS, but how many people know about hepatitis C? Perhaps we should.

Hepatitis C has surpassed HIV as a killer of U.S. adults.

Hepatitis C is a liver infection caused by a virus of the same name that is usually passed through contact with infected blood. An estimated 75 to 85 percent of people with hepatitis C develop chronic infections which can eventually cause serious diseases like cirrhosis and liver cancer.

Researchers at the Centers for Disease Control and Prevention (CDC) found that by 2007, hepatitis C was killing more Americans than HIV — the virus that causes AIDS.

Of the estimated 3.2 million Americans with chronic hepatitis infection, about half of them don’t know it because, in most cases, the initial infection causes no symptoms. Instead, the virus silently damages the liver over the years.

The group at highest risk is baby boomers.