The Infected

Zombies attack the player and become more powerful as the days progress. Here are all the types:

Zombies
Appear all the time.

Stats
Damage: 15

Delay: 1.5 sec.

HP: 100

Spitters
Appear all the time but are more rare than regular zombies.

Stats
Damage: 15

Delay: 1.5 sec.

HP: 125

Special: Can spit goo for a ranged attack.

Damage (Goo Spit): 10

Delay (Goo Spit): 1 sec.

Zombie Goats
Start appearing rarely on Day 4.

Stats
Damage: 40

Delay: No delay, but Zombie Goats ragdoll when they headbutt you, causing a small delay for them to get back up again for the next attack.

HP: 100

Special: Is extremely fast and makes you ragdoll when it headbutts you.

Flamethrower Elephant Zombies
Start appearing on Day 6.

Stats
Damage: No damage, but sets you on fire.

Delay: No delay, FEZs constantly spray flame.

HP: 400

Special: It spews flame that sets you on fire, but it is slower than other zombies.

Naked Russian Guys
Start appearing very rarely on Day 7.

Stats
Damage: See Special.

Delay: See Special.

HP: See Special.

Special: The only way to kill them is to shoot them with The Heartbreaker, or to drown them. They run up to you extremely quickly, stealing your weapon and using it against you. (Mind Control does nothing in the hands of a NRG)

Ghost Zombies
After Day 8, ghostly versions of other zombies will spawn. The ghostly versions have improved stats. Here is a list of improvements.

Zombies
Have 50% more HP and deal 10 more damage.

Spitters
Have 50% more HP and deal 5 more damage (Spitting attack only)

Zombie Goats
Have 50% more HP and don't ragdoll when they headbutt you.

Flamethrower Elephant Zombies
Have 50% more HP.

Naked Russian Guys
Ghost versions of Naked Russian Guys don't exist.