A pill with the potential for halting the spread of HIV sits at the crux of two important health policy issues: President Donald Trump’s plan to end the epidemic in America by 2030 and increasing pressure on drug companies to reduce prices.

The administration’s plan aims to reduce new HIV infections by 75 percent in the first five years and by 90 percent in the next 10 years, with a particular focus on local “hot spots” for the disease around the country.

It relies on putting a medication that protects people from getting the viral immunodeficiency disease into the hands...