Six million to 7 million people are expected to contract AIDS by 1991, far fewer than previous estimates, because high-risk groups are taking precautions, the World Health Organization said.

In March 1987, WHO said 50 million to 100 million people could be infected by 1991 with the HIV virus that causes acquired immune deficiency syndrome. That report said even that estimate "will be conservative if HIV penetrates and spreads through South America and Asia."In its statement Tuesday, WHO said the worldwide total of officially reported AIDS cases climbed to 129,385 in 142 countries in November, an increase of 4.1 percent from October.

It estimated that the true number of AIDS victims was twice the reported number, and that 5 million to 10 million people worldwide already may be infected with the HIV virus. It estimated an additional 6 million to 7 million would be infected in the next three years.

The virus attacks the body's immune system, leaving victims susceptible to a wide variety of infections and cancers. The disease has no known cure.

AIDS is most often transmitted through sexual contact. Other means of transmission include transfusions of tainted blood or blood products, and the sharing of contaminated hypodermic needles or syringes by drug abusers. AIDS can also be passed from mother to child at or before birth.

WHO has seen "evidence for optimism in the question of people's behavior," said Jonathan Mann, director of the organization's special program on AIDS.

"We have seen dramatic evidence of behavior change" in high-risk groups such as female prostitutes, male homosexuals and intravenous drug users of some countries, Mann told a news conference.

He said, for example, that in San Francisco in 1982, the infection rate for homosexual men was about 15 percent a year. But the figure now is 1 percent because many gay men have begun taking precautions such as using condoms and reducing the number of sex partners, he said.

In Amsterdam, the comparable figure has dropped from 10 percent in 1983 to less than 1 percent, Mann said.

But he cautioned that "the evidence is clear that the danger is widespread and is continuing to spread."

Mann also said WHO is "a bit optimistic" that HIV will spread less than earlier predicted because worldwide education programs have been successful in informing people how to avoid getting AIDS.

"The world two years ago didn't know very much about AIDS," Mann said. But he said surveys in some countries show that at least 95 percent of those questioned know about AIDS and know that it can be transmitted sexually.