To get maximum infection, you need to spit on a survivor. Their icon (above their name, bottom left) will start to glow green. If they're infected for to long, either by not curing themselves or getting spit on 3/4 times, they get injured (should see +30 infection) and will get the Broken status effect next to their icon. This means that can't be healed and they have to cure themselves. Once they get the Broken status symbol (red + with a stripe through it), you can hit them with your m1 and you should earn +200 Purge for max infection. Hooking a survivor doesn't reset their infection, unless they've cured themselves.