World experts are gathering in Washington this week for the International AIDS Conference, which starts this Sunday.  The meeting comes as a Bay Area drugmaker prepares to sell its medication Truvada to non-infected people, as a way of protecting them from HIV.  For AIDS activists and researchers, it's just one step closer to what everybody really wants - a cure, or at least, a vaccine. Susan Buchbinder directs the HIV research section at the San Francisco Department of Public Health.  She's been involved in vaccine trials to fight the virus that causes AIDS.