KIGALI, RWANDA—Malaria, small pox and AIDS are among the many afflictions whose spread has been halted thanks to the World Health Organization's recent installation of hot-air hand dryers in millions of Third World restrooms. "Before these dryers arrived, there was much disease and suffering in my restaurant," said Ngwale Goba, assistant manager of a Kigali-area T.G.I. Friday's. "But now that unsanitary paper hand towels have been eliminated, far fewer of us are dying. There is also less chafing."