Only slightly more than a quarter of Americans infected with the AIDS virus are getting the form of medical care that maximizes their life expectancy, according to a new estimate.

The goal of AIDS treatment is to suppress growth of HIV until the virus is no longer detectable in the bloodstream. Only 28 percent of the 1.2 million people living with HIV in the United States have their “viral load” controlled to that optimal degree, epidemiologists at the Centers for Disease Control and Prevention reported Tuesday.

“We have substantial work ahead to fully realize the benefit of treatment in the United States,” Thomas R. Frieden, the CDC’s director, said in a news briefing two days before World AIDS Day on Thursday.

“It is time to act even more aggressively,” said Jonathan Mermin, the agency’s director of HIV/AIDS prevention. Having such a small fraction of people adequately treated “is not acceptable from a public health, humanitarian or economic perspective,” he said.

The estimate is a new way of looking at the 30-year-old AIDS epidemic. No other country has calculated quality of care by incorporating the experience of everyone infected with the virus, including people who don’t know they have it and those who can’t get or don’t want treatment, a CDC spokeswoman said.

For people enrolled in HIV care, the fraction with a fully suppressed viral load is much higher and more encouraging — about 77 percent.

Several years ago, the CDC endorsed universal HIV testing of Americans. With new research showing that treated people are unlikely to transmit the virus to others, the agency is campaigning for physicians and health departments to make sure that people who are found to be infected get into treatment.

About 80 percent of Americans with HIV know they are infected. About 20 percent are unaware — a situation that makes it much more likely they will transmit the virus to others.

Slightly more than 75 percent of people who receive an HIV diagnosis are “linked to care” within four months, but only 50 percent stay in care. The CDC analysis didn’t address why so many stop treatment.

High rates of attrition from treatment are common in medicine. Reluctance to take pills, drug side effects, inconvenience, expense and denial are all reasons people abandon medical care. Mermin noted that “we know from other studies that 32 percent of Latinos, 21 percent of blacks and 16 percent of whites are uninsured, potentially creating an obstacle” to HIV treatment.

Of people remaining in care, 89 percent were prescribed antiretroviral therapy, which consists of three or more drugs that prevent the virus from replicating. Of that group, 77 percent had a fully suppressed viral load the last time they were tested.

In the United States, the average person with HIV survives about 11 years if not treated. (Survival is shorter in many developing countries.) How long people can expect to live with optimal care is uncertain, although it appears to be decades.

A study published this month projected that a man who acquired HIV through homosexual intercourse at age 30 and started treatment before significant damage to his immune system has a life expectancy of 75 years. The infection would be responsible for the loss of about seven years of life, which the researchers said was comparable to the effects of cigarette smoking.

Earlier studies have estimated a somewhat greater loss of longevity from HIV infection. Most projections show that people who don’t start treatment until they’ve suffered immune damage will have even shorter lives.

The new report, published in the CDC’s Morbidity and Mortality Weekly Report, is based on various surveys that monitor HIV testing, patient behavior and treatment success around the country. The data reveal significant differences by age, sex and ethnic group.

For example, 76 percent of HIV-positive people ages 18 to 24 who are in medical care are prescribed antiretrovirals, compared with 92 percent of people 55 and older. Ninety-two percent of whites are prescribed the drugs, compared with 89 percent of Hispanics and 86 percent of blacks. Eighty-four percent of whites achieve full suppression of viral load in their bloodstreams, compared with 79 percent of Hispanics and 70 percent of blacks.

Slightly fewer women (86 percent) than men (90 percent) are prescribed antiretroviral therapy, and slightly fewer women (71 percent) than men (79 percent) achieve viral suppression. A survey of care in 23 different areas around the country found that only 45 percent of HIV patients reported having been counseled in the previous year about ways to lower their risk of transmitting the virus to others. The rate of counseling was higher in younger people than old, and higher in blacks and Hispanics than whites.

On Tuesday, the CDC also announced a five-year, $359 million annual round of funding to state and city health departments where HIV infection is most prevalent. Three-quarters of each grant must go to four specific activities, one of which is to get infected people into care and help keep them there.

The CDC also announced a $2.4 million campaign to urge young black gay men to get tested for HIV, as they are the risk group in which the infection rate is still rising.