SAN FRANCISCO (AP) _ Scientists have no cure in sight for AIDS, and people at low risk for the disease should not smugly comfort themselves with the thought they will be unaffected, Surgeon General C. Everett Koop said.

By 1991 it could cost Americans as much as $16 billion to cope with AIDS, ''the most vicious of infectious diseases in the history of the race,'' Koop told 1,300 public health executives, staffers and medical workers attending the National AIDS Conference on Friday.

''We eventually will have something that will resemble a vaccine that is safe, effective and available,'' but ''at the moment death is the partner of most people with AIDS ... an enormous human catastrophe,'' he said.

Education on all levels of society remains the chief weapon to fight acquired immune deficiency syndrome, which destroys the body's ability to fight disease, Koop said.

The Centers for Disease Control in Atlanta predicted that by 1991, 270,000 people in the United States will have the disease. Since 1981 the disease has afflicted more than 43,000 Americans, of whom 25,000 have died, Koop said. The chief victims have been homosexual and bisexual males and intravenous drug users.

San Francisco city epidemiologist Dr. George Lemp told the conference Thursday that the median length of survival after patients are first diagnosed with AIDS has risen from 10.2 months in 1981, to 14.4 months in 1986. Lemp was unable to explain it.

''This is the first time we've seen improved survival for San Francisco,'' he said. Although he had no projections, he said, ''All we can say is that it's going to be better.''

But the figure has no bearing on the overall survival rate of people with AIDS, Lemp said.

The disease is virtually 100 percent fatal, Koop said.

''We have only a few drugs in the market, AZT the most prominent among them, and these drugs help prolong life a little bit, a few months, maybe a year or so,'' he said. ''But we have nothing at all that will safely and effectively stop the disease in its tracks - nothing.''