For once, there is some good news to report about a blood-borne virus that has infected 4 million Americans and 170 million people worldwide.

The disease, hepatitis C, will eventually debilitate the livers of many of its sufferers, but new cases of it have declined 80 percent since the virus was identified in 1988 and blood banks started screening for contaminated donations four years later.

But -- and this is no small but -- the annual death toll from the long-term consequences of this infection is 10,000 a year in the United States, and scientists expect deaths to triple by 2010 before that statistic begins to decline, unless new treatments are developed to eliminate the virus or at least keep its complications at bay indefinitely.

Several such treatments are being studied, and expert hope they will work as well as those that have radically improved the control of HIV infections. If their early promise holds up in clinical trials, most hepatitis C infections may be cured or at least rendered virtually harmless. Current therapies are lengthy, expensive and can cause devastating side effects. Further, they work in only slightly more than half the patients.

Experts have learned enough about the virus and how it is transmitted to alert those at risk of the need to be tested, to take steps that can forestall complications and to prevent transmission to others.

Unlike HIV, the hepatitis C virus is rarely transmitted through sexual contact. Its primary route to a new bloodstream has been through contaminated needles shared by drug users and by blood transfusions. People with hemophilia and others who received blood products before the testing for the virus began may also be infected.

Low rates of transmission affect health care workers exposed to contaminated blood through needle-stick accidents, men who have sex with men and babies born to infected women.

Fatal cases have resulted from organs inadvertently transplanted from a contaminated donor.

Household contacts and sexual partners in monogamous relationships are rarely affected. But people who engage in high-risk sexual behavior with multiple partners and people who have sexually transmitted diseases face increased risk.

Although those receiving tattoos and body piercings in other countries can be at risk, there is as yet no evidence for transmission by those routes in the United States.

A blood test for the virus relies on the presence of antibodies to it, but antibodies may not appear for weeks after the infection. A more sensitive genetic test can detect the presence of the virus itself.

Testing is recommended for people who have had blood transfusions or organ transplants before July 1992 or were treated for clotting problems with blood products made before 1987, those who have been on long-term kidney dialysis and those who have injected street drugs, even once many years ago.

Not everyone infected becomes ill. Some people seem to eliminate the virus, and a chronic infection never develops.

Others who remain chronically infected may be free of symptoms indefinitely.

In most cases, however, as with HIV, the virus can linger in the body for a long time -- even decades -- before symptoms of liver damage appear.

The most serious consequences are severe cirrhosis, a scarring of the liver, liver failure and liver cancer, which have made hepatitis C the leading reason for liver transplants.

Symptoms, when they appear, are usually mild, intermittent and easily attributed to other causes. The symptoms may include fatigue, nausea, poor appetite, muscle and joint pains and mild discomfort or tenderness in the right upper abdomen.

Those who develop cirrhosis or severe liver disease may, in addition to complaining about those symptoms, experience weight loss, itching, dark urine, fluid retention and abdominal swelling.

No vaccine against the virus has been developed, and prospects for one are not promising because there are at least six major genetic types and more than 50 subtypes of the virus. And, it changes rapidly. The possibility of a vaccine depends on finding an exposed part of the virus that remains stable even as its protein coat mutates.

The main goal of treatment is to eradicate the virus to prevent progressive liver disease. Existing therapies are most effective in patients with Genotypes 2 and 3, which represent about 25 percent of patients in the United States. The most common ones, Genotypes 1a and 1b, affect about 75 percent of patients and are the most difficult to treat.

Two main therapies have been developed. One involves injections of interferon, usually long-acting pegylated interferon, which is injected weekly, and the other an oral antiviral drug called ribavirin.

Therapy is most successful when the treatments are used simultaneously. But each can cause serious problems in certain patients. Interferon should not be prescribed for people with serious psychiatric illness, unstable heart disease or poorly controlled diabetes. People with anemias, heart disease, stroke and kidney disease should avoid ribavirin, as should pregnant women.

Patients with the Type 1 virus are treated for 48 weeks; those with Types 2 and 3 do well with 24 weeks. The combination therapy is effective in slightly more than half the cases, in 42 percent of those with Type 1 and 80 percent for those with Types 2 or 3.

The side effects can be quite miserable, at least at the outset. But they subside with time and disappear when the treatment ends. Patients report that the drugs commonly cause flulike symptoms. They can seriously disrupt sleep and create havoc with sexual response and personality.

People tend to become irritable, forgetful and seriously depressed, and they may lose considerable weight. Even when treatment seems to have eliminated the virus, it can sometimes rebound, requiring a second round of therapy.

While some experts recommend that everyone who has chronic hepatitis C infection be treated, others suggest that each patient, in consultation with physicians, carefully weigh the likelihood that the disease will progress and the benefits and risks of therapy, as well as its considerable cost.

In a recent article in The Journal of the American Medical Association, Dr. Joshua A. Salomon and colleagues at the Harvard Center for Population and Development Studies noted that "30 percent to 70 percent of infected individuals may never progress to cirrhosis before dying from other causes."

The authors further pointed out that progression of the infection was highly variable and unpredictable. The probability of developing cirrhosis over 30 years ranges from 13 to 46 percent for men and 1 to 29 percent for women, they stated. Also, the progression of the infection to serious liver disease is less common among patients who are infected when they are young. They seem better able to fend off the virus or keep it under wraps.

With or without treatment, people infected with the virus should take steps to protect their livers from further damage. The steps include avoiding alcohol, getting vaccinated for hepatitis A and consulting physicians before taking any new medicines, including over-the-counter and herbal remedies