Dr. Robert Grant developed a treatment — a daily pill known as pre-exposure prophylaxis, or PrEP — that could stop the AIDS crisis. We look at why that hasn’t happened. Guests: Dr. Grant, who has been working on H.I.V. treatment and prevention for over 30 years, and Donald G. McNeil Jr., a science and health reporter for The New York Times. For more information on today’s episode, visit nytimes.com/thedaily.