(FORTUNE Magazine) – It's a blood-borne disease, transmitted through transfusions, sex, and shared needles. And it's a silent disease; people can go years before developing any symptoms.

HIV? Certainly. But also hepatitis--specifically, hepatitis B and C, the disease's chronic viral strains that afflict more than 350 million people worldwide. In the U.S. the C strain has been spreading by 150,000 new cases annually, highlighted by the liver transplant case of legendary baseball slugger/sipper Mickey Mantle. Other famously afflicted patients include country singer Naomi Judd and Congressman Joe Moakley (D-Massachusetts), a B strain victim.

Although the B strain has reached epidemic proportions in Asia, where an estimated 230 million people are carriers, in the U.S. hepatitis C is the greater menace. The germ infiltrated the blood supply undetected until 1990, and it mutates rapidly. There are roughly 4.7 million chronic viral hepatitis carriers in America, compared with a million infected with HIV. About 16,000 die from the disease each year, victims mainly of cirrhosis or liver cancer, which is about 50 times more prevalent in hepatitis sufferers.

For drugmakers there's a big opportunity in detection and cures. Chronic viral hepatitis, at $2.4 billion, is the second-largest infectious-disease market worldwide after malaria. The market has been growing 25% a year. SmithKline Beecham sold $583 million worth of B strain vaccine last year; Merck did $205 million in vaccine sales.

Companies such as Amgen, Glaxo Wellcome, Protein Design Labs, and SciClone Pharmaceuticals are working on hepatitis drugs. The current main treatment is alpha interferon, an immune system booster. Schering Plough is the only company approved to sell that drug in the U.S. to treat hepatitis. But alpha interferon has numerous side effects--fever, depression, diarrhea, and hair loss. At best, only 25% of patients with the virus have long-term success.

- Justin Martin