LOS ANGELES, NOV. 8 -- Basketball star Earvin "Magic" Johnson contracted the potentially deadly AIDS virus from a "heterosexual transmission," an official of the Los Angeles Lakers said today. The official, however, could not pinpoint either a specific event or specific partner responsible for the transmission of the virus.

Johnson, 32, shocked the nation Thursday when he announced he had tested positive for HIV and was retiring from basketball immediately after 12 dazzling seasons with the Lakers. He said he would become "a spokesman for the virus" and encourage people, particularly young people, to engage in "safe sex."

Bob Steiner, public relations director for California Sports Inc., the Lakers' parent company, said Johnson had contracted the virus in a heterosexual relationship. It also was announced that Johnson's wife, Cookie, is seven weeks pregnant. Johnson said Thursday that she does not have HIV. However, the possibility remains that both the wife and the embryo could contract HIV later, according to medical authorities.

While the AIDS virus can be transmitted heterosexually -- and thousands of AIDS cases in the United States have resulted from heterosexual contact -- the risk of it happening is far lower than in homosexual contact, largely because the fraction of heterosexuals now infected is very much smaller than that of homosexuals.

In other words, the odds of finding a partner who is infected are quite low -- with the major exception of prostitutes, a large percentage of whom are infected. It is also believed that the odds of an infected man passing the virus to a woman are far greater than those of an infected woman passing it to a man. If a man has another venereal disease, such as herpes, his odds of being infected are vastly higher.

At his news conference Thursday Johnson said: "I think we sometimes think only gay people can get it; it's not going to happen to me. And here I am saying that it can happen to anybody, even me, Magic Johnson."

According to NBA sources, the Lakers knew "about two weeks ago" that something was wrong with Johnson. When he was informed Thursday that he had tested positive for the AIDS virus, he immediately announced that he would hold a news conference. A league official expressed concern to Johnson, fearing that the questions would be intrusive and painful for Johnson to answer. But Johnson insisted otherwise.

Tonight Johnson maintained a high profile, appearing on "The Arsenio Hall Show." He came on stage at the beginning of the show with Hall, and he received an extended standing ovation.

Johnson, with his trademark smile as broad as ever, tried to quiet the crowd by calling "time out," but then chants of "Magic, Magic" rang out even louder.

Hall asked Johnson why he was on the show. "Having HIV virus, I want everybody to practice safe sex, be aware what's going on," Johnson said. " . . . I want to educate the public. We don't have to run from it, be ashamed."

Since he does not yet have AIDS, Johnson told Hall why he decided to retire from basketball immediately: "If I still continued to play, put stress on my mind and body, the virus would have overtaken me three, four, five years from now. Now I have a chance to live longer, if I eat right, take my medicine."

Johnson's dramatic disclosure on Thursday triggered a national outpouring of sympathy, and a flood of calls to health centers and AIDS hotlines by people who asked to be tested for HIV.

"Our phone's been ringing off the hook," said the Rev. Carl Bean, founder and chief executive officer of the Minority AIDS Project here. "It's proven that he {Johnson} is truly a champion. He's made a big difference by proving that HIV is not discriminatory."

Throughout Southern California, people paused in shock and sympathy over Johnson's disclosure. Condom sales soared. Local sports announcers were near tears as they reported the news Thursday night. Today the steps at Los Angeles City Hall, where Johnson stood after leading the Lakers to five National Basketball Association championships, were renamed in his honor.

"His honesty and courage will be the shot heard 'round the world in the battle against AIDS," said Councilman Joel Wachs.

Esther Hayes, an expert in AIDS and associate dean at UCLA, said she counseled Lakers players before the news conference for about 40 minutes at the team's invitation, explaining to them how HIV is transmitted.

"At first they were overwhelmed and very quiet," Hayes said. "They have a close relationship and are very involved with one another emotionally. Then they began to ask questions. They wanted to know if one player could give it to another from physical contact, which I told them is very unlikely. They wanted to know if they could get it from sweat, and I told them sweat is just a salt solution and it's transmitted by white blood cells."

Gradually, Hayes said, the players asked questions about sex relationships. She said she told them about the importance of using condoms and said that "multiple sexual partners puts you at risk."

Hayes said that because of the high regard that people hold for Johnson his coming forward is likely to have a positive impact in demonstrating that anyone can get HIV and in encouraging those with risky lifestyles to be tested for it.

"This is a tragedy for Mr. Johnson but a potential benefit for victims of the epidemic," she said.

Lakers players were somber at Los Angeles International Airport before leaving to play a game against the Phoenix Suns. Byron Scott, the guard who teamed with Johnson in the backcourt, said that no one felt like playing. But Lakers Coach Mike Dunleavy said the team would play "because it's what Magic would want us to do."

Dunleavy said about Johnson, "He'll always be a Laker."

Bulls superstar Michael Jordan said today at a news conference in Chicago he will join his friend Johnson in helping to educate Americans about AIDS. Jordan recounted that Johnson had called him on his car phone to tell him the sad news.

"I'm almost driving off the road because I couldn't handle it as well as he could," Jordan said.

Vice President Dan Quayle, in Los Angeles for a round of political appearances, said that the Food and Drug Administration is on the verge of announcing changes in procedures that will speed the approval of new drugs, some of them to deal with AIDS. Quayle called Johnson "a true champion" but said his approach to halting the spread of HIV would stress abstinence rather than "safe sex."

The alarm triggered by Johnson's announcement revealed the ignorance that many people have about the way HIV is transmitted.

Charles Medrano, assistant to the director of health education at the University of Southern California, said a student was overheard saying that he had shaken hands with Johnson and wondered if he could contract the virus because of it.

This apparently was not an isolated instance. Medrano said that calls at the health center for students wanting HIV tests had tripled but that a number of callers wanted to know if they could contract the virus through handshaking or casual contact in a swimming pool.

Such contacts do not cause HIV. The virus almost always is spread by intimate sexual contact, the sharing of needles by intravenous drug users, blood transfusions or from a mother to a fetus.

The first report that Johnson had contracted the virus through heterosexual contact appeared in today's Orange County Register. The newspaper quoted Michael Mellman, who is Johnson's doctor and a Lakers team physician.

"This is a heterosexual individual who was infected through heterosexual activity," Mellman was quoted as saying.

Stars in other sports expressed their sympathy for Johnson today. The Associated Press quoted Wayne Gretzky of the Los Angeles Kings hockey team as saying, "Our prayers are with him. I think people think superstars are invincible, tend to put them on a little bit of a pedestal. But we bleed, sweat, cry and have ups and downs just like everyone else."

Special correspondent Leef Smith contributed to this report.