Researchers report they are confounded that deaths from a preventable, curable disease hit an all-time high in 2014.

ATLANTA,  (UPI) — Despite hepatitis C being preventable and curable, the number of people who died after contracting it hit an all-time high in 2014, baffling researchers at the U.S. Centers for Disease Control and Prevention.

CDC researchers found nearly half of the 3.5 million people with hepatitis C are unaware of their infection, and the numbers are rising among people at high risk because of a lack of awareness.

Hepatitis C is caused by a virus that attacks the liver, and while some people have symptoms most do not, the CDC reports.

A large portion of people infected with the virus are baby boomers infected as a result of medical procedures involving blood transfusions before techniques to prevent the spread of disease were in place.

The researchers also point to an increase in the spread of disease among people who inject drugs, because of sharing needles or other unsafe practices, as a potential cause for the increase in deaths from the virus.

“Because hepatitis C often has few noticeable symptoms, the number of new cases is likely much higher than what is reported,” Dr. John Ward, director of the CDC’s Division of Viral Hepatitis, said in a press release. “Due to limited screening and underreporting, we estimate the number of new infections is closer to 30,000 per year. We must act now to diagnose and treat hidden infections before they become deadly and to prevent new infections.”

For the new study, published in the journal Clinical Infectious Diseases, researchers analyzed death certificates for cause of death.

The certificates show 19,659 people died for reasons linked to hepatitis C, though they note death certificates often underreport the disease as a cause for death. The number of people who have died from the disease between 2003 and 2013 has now surpassed 60 other infectious diseases combined.

Of an estimated 3.5 million Americans who have the disease, the researchers said about half don’t know they have an infection, underscoring the need for improved awareness, screening and treatment for it.

“Why are so many Americans dying of this preventable, curable disease?” said Dr. Jonathan Mermin, director of the CDC’s National Center for HIV and AIDS, Viral Hepatitis, STD and TB Prevention. “Once hepatitis C testing and treatment are as routine as they are for high cholesterol and colon cancer, we will see people living the long, healthy lives they deserve.”