Since acquired immune deficiency syndrome was first recognized in 1981, the grim totals have risen at an accelerating rate and now are doubling every year. The latest U.S. toll: 12,736 known cases, 6,376 known deaths. In the District: 207 cases, 103 deaths.

For those who have it, for those who have been exposed to it and for those at risk of getting it, AIDS has become another four-letter word for fear.

More than 1 million Americans may have been exposed to the AIDS virus; about one in 10 of those will come down with AIDS. A person carrying the AIDS virus carries it for life, and most who are infected do not know it yet.

What sets AIDS apart from other epidemics is not sheer numbers -- influenza will kill many more Americans this year. But as the numbers have grown, their pattern has remained alarmingly constant: Half of AIDS' known victims are dying, the other half are dead.

AIDS is not a single disease but a lethal condition of risk. The AIDS virus attacks and disables the body's immune system, the very cells that normally protect against infections. A patient with AIDS is vulnerable to a long list of "opportunistic" infections that the immune system normally would fight off harmlessly. Median survival is about one year.

The AIDS virus has been found in blood, semen, saliva and tears, but there is no evidence it can be transmitted by casual contact such as shaking hands, coughing or being in the same room. As infectious diseases go, AIDS is relatively hard to contract. It is spread almost exclusively by exchange of bodily fluids through sexual relations or sharing of intravenous needles.

Ninety percent of the reported cases of AIDS fall into two groups: gay or bisexual men and intravenous drug users. This has colored public reaction to the disease from the start, heightening the fear and, critics say, delaying mobilization of a public health effort against AIDS.

It also has led some to call AIDS "the plague of the 20th century" and compare AIDS victims to lepers. Besides the physical and mental ravages of their illness, patients with AIDS often face poverty -- medical costs alone average $114,000 per case -- and the isolation and stigma of being treated as outcasts.

Anyone with AIDS has experienced or heard the horror stories -- of patients disowned by their families, fired from jobs, shunned by friends, denied care in hospitals.

"We're going to look back at this in five years," says Caitlin Ryan, program director of the AIDS Education Fund at Whitman-Walker Clinic in the District, "and we're going to be shocked at what did and did not happen."

In 1983, Health and Human Services Secretary Margaret Heckler called AIDS the Public Health Service's "number one priority," but the Reagan administration repeatedly cut that agency's budget requests until recently, when it asked Congress to boost appropriations for AIDS research in fiscal 1986.

AIDS is an enormously complex illness about which much remains unknown, including its origin. The best guess is that it's a mutant virus that originated in central Africa in the green monkey and spread to humans, who carried it to the Caribbean and North America in the late 1970s.

It came to light nearly five years ago, when doctors in New York, San Francisco and Los Angeles began seeing surprising clusters of cases of two rare diseases -- Kaposi's sarcoma and pneumocystis carinii pneumonia -- in previously healthy young gay men. Kaposi's is a cancer found almost exclusively in elderly males of Mediterranean descent, and pneumocystis is usually found only in people with suppressed immune systems, such as cancer or transplant patients.

On the basis of such reports, the Centers for Disease Control announced in mid-1981 the existence of a new disease, and the AIDS epidemic was under way. Two years later, scientists identified the virus responsible -- dubbed HTLV-3 -- and have since developed a test indicating exposure to it. That test is used to screen blood donations.

But still there is no cure. An AIDS vaccine, scientists say, remains a long shot. The best hope is to prevent the spread of the disease by screening blood transfusions, discouraging intravenous drug abuse and encouraging safe sex practices among high-risk groups.

"We would be short-sighted," wrote Dr. Edward Brandt Jr., former assistant secretary for health, in the foreword to a new textbook on AIDS, "if we did not heed a key lesson of this epidemic -- namely, that science alone is not enough."