What is out there.
Most of them are walkers. The rest are the reason you check corners, and the reason a full magazine is not always the answer.
Fighting them.
Noise pulls them
Gunfire carries. A suppressor buys you the difference between one walker and a street of them.
Every bite counts
Damage is one problem, infection is the other. Armour with bite protection is worth more than raw armour value.
Numbers beat tiers
A tier-one walker on its own is nothing. Twenty of them between you and the door is how most runs end.
They evolve
The world's infection deepens over time, and the roster gets worse
with it. Check where it stands with /zmutation.