The toughest battlefield in the war on AIDS will be Africa, but the last skirmishes may be fought in the back alleys where drug addicts congregate, the head of the World Health Organization says.

"A health problem is never just a medical problem," Dr. Halfdan T. Mahler, director-general of WHO, said in an interview. "AIDS is just as much a political problem. It's a social problem. It's an economic problem."Mahler, a craggy-featured Dane, speaks of fighting disease with a missionary zeal. He says he refuses to be daunted by the challenge of acquired immune deficiency syndrome.

"If you get overwhelmed, then you lose your imagination," he said during a visit to U.N. headquarters this week.

The Geneva-based WHO estimates there are about 150,000 cases of AIDS worldwide and an additional 5 million to 10 million people infected with the AIDS virus. It says 129 countries have reported the disease.

Mahler said AIDS is spreading fast in urban Africa south of the Sahara, a region where economic problems have made even the fight against curable diseases difficult.

"Africa has been more underprivileged and more overtraumatized by history than any other place," Mahler said. "Obviously it is a continent where development has gone wrong. But I take AIDS as a challenge in this difficult situation."

There is no known cure or vaccine for AIDS, a viral disease that destroys the body's immune system and leaves victims susceptible to a wide variety of infections and cancers. It is spread through sexual contact, tainted blood, sharing of contaminated hypodermic needles or syringes by drug abusers, or passed on by infected mothers to their children at or before birth.

Mahler praised the efforts of sub-Saharan nations to develop national programs against AIDS with the help of WHO.

"We have truely been able to set in motion programs that, I believe, in spite of the absence of vaccines and drugs, will bear fruit over the next four, five, six years," he said.

Intravenous drug abusers, however, may prove to be the most intractable problem, he said, partly because they tend to be self-destructive and are less likely to take the kinds of precautions that can stem AIDS. Compounding the problem is the worldwide failure to curb drug use, he said.