South Africa (study was in SA and Uganda) has an adult HIV prevalence of 18.3% and 210k new infections per year. It is easy to select a high risk group in which you would expect to see new HIV infections during the course of the study without intervention.

They don't need to ensure that all test groups are exposed to people with HIV. There is already a known risk factor for the specific population. All they need to do is see if the people who received the medication had more/fewer/similar infection rates as those who didn't.

Think about it like studies on which cars perform better in crashes. They don't need to have people drive more wrecklessly to determine if the car is safer. They just need to look at the expected risk compared to the outcomes of the people who drive that car. They are already doing the risky thing.

> So, my thinking was how they ensure that all test groups are sexually exposed to other people with HIV, for the trial to be effective.

They don't. Some people will organically have sex with people with HIV, and some will not. Your study just needs to recruit enough participants that it is likely some will. Your study absolutely does not tell people to deliberately have sex with HIV+ partners.

I think it is like coin flips, do enough of them and the total heads or tails gets very predictable even though individual flips are not.