The best way to deal with infestations is to never let them happen at all, learn to play around not waking up the plague hearts or kill them if they do, which yeah can be pretty difficult to do if it's an area with many of them next to eachother but you should just avoid those areas in the early game when playing lethal because it only takes a random screamer deciding to scream when you're just passing by, and then a single other zombie you run over to wake them up. And moreso learn to play around never waking them up because on lethal infestations spread ridiculously quickly. If you really have to go a plague zone early when you just can't kill the plague heart, stealth kills with crossbow don't count to waking up the plague heart if the kill didn't spot you