From the start, Surgeon General C. Everett Koop had warned the students of Cardozo High School that he would be blunt.

"The facts about AIDS are not very nice," the nation's top doctor told students assembled at the Northwest Washington school yesterday in one of his first AIDS addresses to high school youngsters. With that, he launched into a brisk and frank description of the fatal disease and its especially devastating rate of growth in the black community.

Everything went smoothly until Koop described precisely how AIDS is contracted. While any sexual intercourse with someone who has AIDS is risky, anal sex is especially dangerous because both semen and blood are transferred to another person.

"The human rectum was just not designed for that kind of thing," Koop said.

The well-behaved, obviously fascinated audience of more than 700 students had reached the frontier of frankness. They shifted about in their seats, gasped, coughed, giggled.

Koop ignored the discomfort and charged on. He had information to impart, misconceptions to destroy.

"We are fighting a deadly disease," he said. "We are not fighting the people who have that disease. Homosexuality does not produce AIDS."

That comment also drew muttering and wisecracks. But Koop's next message silenced the auditorium.

"I'm sorry to say minorities in America have a disproportionate number of cases of AIDS," Koop said. "One of every eight Americans is black. Two of eight Americans with AIDS are black. AIDS is dangerous. AIDS is fatal. AIDS is spreading. AIDS is spreading fastest among black Americans.

"Government is not going to solve the AIDS problem. The AIDS problem in the black community can only be solved by people who live in that community."

That's why Koop has expanded his speaking schedule to include virtually all-black high schools such as Cardozo. Education, he said, is the best defense against the disease.

Although Koop said he was impressed by the intelligent questions students asked after his speech, he said many young people still cling to rumors and misinformation about AIDS.

"One guy in my class wouldn't even come to the assembly today because he thought they were going to bring in people who have AIDS," said Michael Pinkney, a 10th grader. "Everybody's real scared of getting attacked by a person with AIDS."

"A lot of kids think you can get it all different kinds of ways," said senior Bridget Ellison. "I particularly think blacks don't know too much about it."

Students questioned Koop about the incubation period of the virus (an average of 5.5 years), the life expectancy of a victim (most die within two years of contracting AIDS) and when there may be a cure ("I would be very much surprised if we had a vaccine in this century," Koop said).

Senior Curtis Lane wanted to know how many people have gotten AIDS even after they used a condom. Koop said he had no statistics, but warned that even condoms do not provide complete protection from the disease. "Many people who use condoms don't know how to put them on," he said. Then he described exactly when to put them on and take them off.

The only certain protection against AIDS "is not to have sex at all." That idea drew laughs and even a few hoots. Koop did not crack a smile. "The next best thing is to find the one right person . . . what we call monogamy. When you have sex with someone, you are having sex with everyone that he or she has had sex with in the past."

While most students said they don't know anyone who has stopped having sex because of AIDS, Timothy Braxton, a Cardozo senior, said, "AIDS made me calm down a whole lot. Sex is still pretty high here, but it's kind of slowing down. You tell that general he's a cool dude for coming down here."