Second of two articles. Read the first one here.

Last week, the High Court of England and Wales announced a momentous decision: It invalidated the pharmaceutical company Gilead’s patent on Truvada, opening the way to generic competition.

Truvada, a combination of two drugs, is one of the world’s most-used H.I.V. medicines. For treating H.I.V., it’s used along with a third drug. But many H.I.V.-negative people also take Truvada daily as a preventive. That’s called pre-exposure prophylaxis, or PrEP.

In the United States, Truvada is available only as a brand-name drug. It costs $20,000 a year.

Here’s how it will work in Britain’s National Health Service, according to Dr. Andrew Hill, a senior research fellow at Liverpool University who studies the cost of medicines. “The N.H.S. will say to a group of generic companies: ‘We need PrEP for 20,000 people. Give us your best price.’” The cost of making PrEP is $55 per year, Dr. Hill said. He believes that the generic will sell for between $100 and $200.

All over the world, more and more people are taking H.I.V. drugs. These medicines are very good at their job — keeping people healthy and noncontagious — so most patients will take them until they die of something that isn’t H.I.V. Patients are also starting earlier on antiretroviral therapy; the new recommendation is to start immediately upon diagnosis. And now with PrEP, a potentially enormous new group of patients has arisen: H.I.V.-negative people who are at risk for catching the virus.