Zombie nightstalker/doc

Zombie nightstalkers are invisible at pitch-black darkness and night vision doesn't aid you spot them until they are adjacent to you. Flashlights and other light-emitting objects help you detect these zombies. Infrared vision also helps you see them without attracting additional hordes at night.

They are rare and appear suddenly. But don't be afraid to take them on IF you have an advanced melee skill, wear reasonable armor (diificult in summer) and your counter attacks have a good output of 30+ damage. (If you don't fullfill these three conditions: RUN and hope you can escape.)