Forty years ago, a beloved neighbor was bedridden for weeks at a time with a mysterious ailment. She knew only that it involved her liver and that she must never drink alcohol, which would make things worse.

It was decades before the cause of these debilitating flare-ups was discovered: a viral infection at first called non-A, non-B hepatitis, then properly identified in 1989 as hepatitis C. The apparent source of her infection was a blood transfusion she had received decades earlier.

A screening test was soon developed, making it possible to check all blood products for the hepatitis C virus. But that by no means put an end to the infection. Transmission persists today, commonly the result of intravenous drug abuse with shared needles, sexual and especially anal intercourse, and, among health care workers, accidental needlesticks or other contact with infected blood.

鈥淎n estimated 3.2 million people in the United States are infected, but the vast majority of them don鈥檛 know it,鈥 Dr. Mark S. Sulkowski, a liver specialist at Johns Hopkins University School of Medicine, said in an interview.

For several decades, only people at high risk for the infection were advised to be screened for it. That meant anyone who had ever injected illegal drugs; recipients of blood transfusions or organ transplants before 1992, or of clotting factor concentrates made before 1987; children born to infected mothers; patients who underwent long-term kidney dialysis; anyone infected with H.I.V. or with symptoms of liver disease or an abnormal liver enzyme test; organ transplant recipients whose donors were later found to have the virus; and health care workers possibly exposed to infected blood.

But even this wide net has missed huge numbers of infected individuals, Dr. Sulkowski said.

Many at high risk are reluctant to identify themselves for screening. Others are unaware that they might be infected, including those exposed as infants or children. In more than half of infected people, the abnormality does not show up in routine blood tests until serious damage has occurred. A chronic infection can cause cirrhosis and liver cancer, and often necessitates a liver transplant.

Recognizing that deaths from hepatitis C are rising and more than three-fourths of infections are being diagnosed in baby boomers, the Centers for Disease Control and Prevention now recommends that everyone born from 1945 through 1965 be screened for the virus.

But what about other people who are walking around with undiagnosed hepatitis C infections? Should they wait until their livers are seriously damaged?

鈥淚 would recommend that everyone who comes in for a checkup be screened for hepatitis C,鈥 said Dr. Hillel Tobias, a liver specialist at New York University Medical Center. 鈥淚t can be added to a blood test and is covered by insurance.鈥

Thomas Carley, a 39-year-old resident of Mahopac, N.Y., and father of 7-year-old twins, knows the value of early detection. Apparently infected with hepatitis C as a child, he was diagnosed with Stage 4 liver cirrhosis in his 20s and eventually needed a liver transplant.

He is now an enthusiastic volunteer for the American Liver Foundation, which, he said in an interview, 鈥渢aught me everything I needed to know to fight this disease.鈥

In about 20 percent of cases, the virus disappears on its own within six months of the initial infection. But the remaining 80 percent develop into a chronic infection that can slowly destroy the liver.

鈥淭he younger you are when you鈥檙e infected, the longer it takes to develop cirrhosis,鈥 Dr. Tobias said. 鈥淚t could take 25 years or more in someone infected at age 20. But a 50-year-old can develop cirrhosis in just 10 to 15 years.鈥

The earlier an infection is diagnosed and treated, the less likely that liver damage will occur. But even in people who already have cirrhosis, eradicating a hepatitis C infection 鈥渕arkedly reduces the chances of it progressing or of developing liver cancer,鈥 Dr. Tobias said. 鈥淓ven with advanced cirrhosis, people can live longer lives if you get rid of the virus.鈥

Until late last year, the standard treatment for hepatitis C infection was a challenging 48-week regimen of weekly injections with interferon along with one or two oral antiviral drugs, ribavirin and a protease inhibitor. The treatment almost invariably caused fatigue, depression, irritability, nausea and other debilitating side effects, prompting many infected individuals to refuse it unless obvious liver damage had occurred.

But with two newly approved drugs and a few more in the pipeline, a new era in treatment of hepatitis C is at hand. These regimens are more effective at curing patients and generally work much more quickly than previous treatments.

Hepatitis C has a variety of genetic forms 鈥 at least six. Most American patients are infected with genotype 1. The new treatments must be carefully selected for each patient, because some drugs are more effective than others against particular genotypes.

The new drugs, sofosbuvir (Sovaldi) and simeprevir (Olysio), are each approved for use with interferon and ribavirin for treatment of genotype 1 infection. Sovaldi already can be used without injected interferon to treat people infected with genotypes 2 and 3 鈥 about a quarter of all hepatitis C patients in this country.

The Food and Drug Administration is expected to approve an all-oral drug treatment for genotype 1 infection, without interferon, toward the end of this year. But many patients are already taking oral combinations of the newer antivirals, prescribed off-label by their doctors or obtained in clinical trials.

A major study of the new drugs, called the Cosmos trial, found them effective even in patients who could not be cured by previous treatments. One very grateful recipient, John DiFazio, 62, a Vietnam vet and retired firefighter living on Staten Island, said that since the late 1990s he had tried half a dozen different treatments for hepatitis C, all various combinations with interferon, and none had cleared his body of the virus.

He started the new drugs in January and within seven days, his viral count had dropped to 938 per milliliter from 2.8 million. Now six weeks out, it is 104 per milliliter.

鈥淚鈥檝e had no side effects,鈥 Mr. DiFazio said. 鈥淚 can do everything I want to do.鈥

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