When shooting infected enemies, all of a sudden, the bullets from my gun stop having any effect/damage on them, regardless of which gun I use. It's as if my character can no longer deal damage with guns until I die. Melee weapons still work fine, though.

The only way I have found to fix the issue is to log out and log back in.

From what I've observed, it may happen when shooting at very close range (shooting from above a car while surrounded by a large group) or during the infected's attack animations. That's all I've observed so far, but I'm not 100% sure.