AIDS is literally just a diagnosis. They have a chart of component “diseases” and if someone has some number of them, they call their status “AIDS”.

Assuming there actually is some real thing associated with what they call “HIV”, I wouldn’t be shocked if it was just a parasite. Maybe it’s able to eat white blood cells if they don’t attack it fast enough, and so if someone’s white blood cells are already busy with 3 other sepsis-based infections, they don’t have the numbers to take it on, and it’s able to take root.