Virologist Donald Francis, who successfully battled Ebola in the Sudan and smallpox in India, is now taking on the trickiest assignment of his career: an international test of an AIDS vaccine. Dr. Francis announced Wednesday that his company, VaxGen Inc., has won permission from the U.S. Food and Drug Administration to launch the nation's first large-scale test of a vaccine for AIDS prevention.The Phase III trial will include 5,000 volunteers in the U.S. and 2,500 in Thailand and take at least three years to complete.