The Zombie is the main and quite likely the First Enemy Type you will face in your Ventures. They possess a natural weakness to Holy as they bear the Chaos Attribute, due to their decaying claws they may make you bleed when you get hit - Ranging from "that was easy", to slow and hard to take down.
The First of the Zombie variants is the Classic Standard Zombie While they may not be fast, just as with any zombie getting overwhelmed is not ideal as they can quickly kill you. They tend to be fairly slow letting you lead them around pillars or walls to delay them and to help take them down by Aiming for the head.
While not the First of the Zombies, they will be the First to meet you and claw at you if given the opportunity. Fast Zombies are extremely quick in both their lunges and attacks, where they jump great distances to stab into you and deal lots of damage in a quick ravenous flurry of attacks. However due to their impulsiveness to jump at first sight they can be baited into jumping the wrong way or into a line of fire, letting you quickly kill them.
A Combine Soldier that meet a horrific fate and not by the hands of you. These zombies are more Tanky then the Standard Zombie and will often pull out and activate a MK3A2 Grenade to Charge and try to send you both to a flashy but messy end. When they charge be wary as they often drop the Grenade when killed, so be sure to try and Return to sender.
A Tall and Dangerous Monstrosity that can slap you into the air. Tanks may be slow but due to lots of heath and lack of a headshot they can take a lot of damage making them hard to deal with at short notice. Try to lead these titans along large pathways to give yourself plenty of time to chip away at them before they get close.
The Zombie that lost their faith. While these Zombies may be quicker then normal and take a few more rounds to put them down due to their slightly larger heath. As the same with the Standard Zombie, lead them around to delay them and aim for the…"head".
A slow and slumped over zombie that will corner you if able. Lucky for you the Headcrabs have since been trained as pets. So there's no lethal poison that will send you looking for a H.E.V Suit in a hurry. However these zombies are still Tanky with lots of health forcing you to chip away at them.