Zambian President Kenneth D. Kaunda publicly acknowledged for the first time that his son's death last year was caused by AIDS. "How my son got AIDS I do not know," he told reporters at a news conference in Lusaka, the capital. His admission dramatized the severity of the problem of AIDS in his southern African nation. Western diplomats have estimated that about 20% of Zambia's adult urban population is infected with the AIDS virus. Kaunda's son, Masuzgo Gwebe Kaunda, 30, died of liver and kidney failure. Acquired immune deficiency syndrome is an affliction in which a virus attacks the body's immune system. "It does not need my son's death to appeal to the international community to treat the question of AIDS as a world problem," Kaunda said.