It can take decades for the blood-borne virus to cause liver damage and symptoms to emerge, so many people don't know they're harbouring it. Baby boomers account for about two-thirds of the estimated 3.2 million infected Americans.
More than 15,000 Americans die each year from hepatitis C-related illnesses and the number has been growing, according to the Centers for Disease Control and Prevention.
"Unless we take action, we project deaths will increase substantially," said CDC Director Dr. Thomas Frieden, in a call with reporters.
Hepatitis C virus is most commonly spread today through sharing needles to inject drugs. Before widespread screening of blood donations began in 1992, it was also spread through blood transfusions.
The virus can gradually scar the liver and lead to cirrhosis or liver cancer, and is the leading cause of liver transplant. It can trigger damage in other parts of the body, as well.
It's possible some people were infected in ways other than dirty needles or long-ago blood transfusions. Some experts say tattoos, piercings, shared razor blades and toothbrushes, manicures and sniffed cocaine may have caused the virus to spread in some cases.
However it happened, health officials say baby boomers are five times more likely to be infected than other adults.
Officials said they decided to issue the recommendations after seeing the number of Americans dying from hepatitis C-related diseases nearly double from 1999 to 2007.
Another reason: Two drugs hit the market last year that promise to cure many more people than was previously possible.
Previously, testing was recommended only for people considered at highest risk, like current and former injection drug users.
About 3 per cent of baby boomers test positive for the virus, the CDC estimates. Of those, some manage to clear the infection from their bodies without treatment, but still have lingering antibodies that give a positive initial test result. That's why confirmatory tests are needed.
Still, only a quarter of infected people are that lucky. Most have active and dangerous infections, health officials said.
"I have met too many patients who were diagnosed with hepatitis C at the time they developed liver cancer or when they needed a liver transplant," said Dr. Andrew Muir, a Duke University physician who is a leader in an advocacy organization called the National Viral Hepatitis Roundtable.
The CDC call for testing is "a bold and important move," Muir said in a statement.
The recommendation applies to people born from 1945 to 1965 who have not already been tested. They should get a blood test at their next visit to the doctor, Frieden said.
The CDC proposed the new guidelines earlier this year and made them final on Thursday.