BOSTON--An estimated 2.5 million people in North and South America are already infected with the AIDS virus, and some parts of the hemisphere may be facing "a massive epidemic" of AIDS, health experts warned yesterday.

The doctors warned that the epidemic in some Latin and Caribbean nations may become as bad as the AIDS outbreak in Africa, although the severity will vary from country to country.

Their concern, expressed in an editorial in the New England Journal of Medicine, is based in part on the apparent spread of the disease from homosexuals to heterosexuals. This shift will put far more people at risk of AIDS.

The editorial predicted that 500,000 people in the Western Hemisphere are likely to be diagnosed with acquired immune deficiency syndrome by 1992.

"We have great concern that we are starting to see a more heterosexual pattern of spread emerging in these countries," Dr. Thomas C. Quinn said. "Once it becomes an established heterosexual epidemic in those countries, it has a potential for rapidly increasing in sheer numbers, like in Africa."

Quinn, an AIDS expert at the National Institute of Allergy and Infections Diseases, co-authored the editorial with two physicians from the Pan American Health Organization.

A study in the same issue of the journal described the level of AIDS infection in Brazil. Dr. David D. Ho of Cedars-Sinai Medical Center In Los Angeles, senior author of the study, agreed with Quinn's concern.

"We are very fearful of the same situation," Ho said.

In their editorial, the doctors warned, "If HIV-1 infection continues to penetrate the poor and less advantaged populations of Latin America and the Caribbean, there is the potential for a massive epidemic in the Americas that may parallel the situation in Africa, where many cases remain unrecognized and unreported."

In Africa, the disease is spread primarily by heterosexual intercourse and affects men and women in almost equal numbers. However, in the United States, victims are largely male homosexuals and drug abusers, and men outnumber women about 10 to 1.

Quinn noted that the epidemic in most Latin and Caribbean nations is far behind Africa's, but there are troubling signs that it will follow a similar course.

Among these are the relatively high proportion of women victims in some countries. The male to female ratio in French Guiana is 1.5 to 1; Hondurus, 1.7 to 1; Bahamas, 1.8 to 1, and Trinidad, 4 to 1.

Quinn said poverty levels and a high frequency of prostitution in some Latin and Caribbean countries may also contribute to the transmission of AIDS as a heterosexual disease.

He noted that the disease is already well established in some countries. In Haiti, 10 percent of pregnant women are infected, the same as in some parts of Africa. There are 379 AIDS cases per million people in the Bahamas and 350 per million in Bermuda, compared with 71 per million in the United States.

However, the epidemic will vary from country to country. He predicted that in Chile, Bolivia and Argentina, among other countries, the spread to heterosexuals will probably be slow, as it as been in the United States.

Quinn said the relatively large numbers of bisexual men in some Latin countries appears to be hastening the spread of the disease from homosexuals to heterosexuals. One of these is Brazil, where Ho's study found that 28 percent of bisexual men and 23 percent of homosexual men were infected with the AIDS virus.

The study found that 9 percent of lower-class female prostitutes in Rio de Janeiro were infected, but none of middle- or upper-class prostitutes tested were infected.

The researchers also found five people who were infected with HIV-2, an apparently less virulent relative of the primary AIDS virus that until now has been confined largely to West Africa.

"The second AIDS virus is spreading to this part of the world, slowly but surely," Ho said.