Board Thread:Metal Gear Solid V/@comment-5.78.86.253-20150920204454/@comment-25366375-20150921034745

He still could have stopped the parasite's capacity to breed, which is precisely what the Wolbachia treatment did. You'll notice that everyone on Mother Base still has the parasite within them, but the Wolbachia meant that the male components turned female, so breeding could not occur.

Quiet, as you discover in Mission 45, refused Wolbachia treatment. Code Talker theorises that it may be because she still wants revenge on Snake and Diamond Dogs.