Runners
Runners are the first and weakest stage of the Infected who have recently entered their transformation. Since the eyes are first targeted by the fungus, runners have poor eyesight, but maintain many human instincts. For example, their moans are still human, just like their overall appearance. A way to tell them apart from normal survivors is their glowing red eyes. Their body language is sluggish while they are passive, suggesting they are resisting the fungus's influence. Sometimes they do not attack at all, even when non-infected individuals are right in front of them. These non-aggressive runners will only attack if attacked first or when an object is thrown at or near them, thus losing control. Much like sneezing, the urge to attack after such interactions happens involuntarily and unwillingly by the host. Runners are confirmed to be able to attack in packs flailing their arms trying to trample the survivor.
They can be attacked and killed from behind if they don't spot you first.
Strategy
Anything works on them, just don't let them swarm you in large numbers. It would be advised to run away if they ever do swarm you, then take out your gun and start shooting. Do not waste time using a knife on them as you will need those for Clickers and possibly Stalkers if in large numbers.
Stalkers
Stalkers are the second stage of the Infected. They have the vision and speed of Runners and a slightly weaker enhanced echolocation of Clickers. The most notable physical traits that define the Stalkers is the distinct croaking noises they make, the beginning of fungal growths on the head and face, the development of pseudo echolocation, and their discretion upon spotting a victim at distance - they will strafe and take cover, eventually closing in on the victim, hence the name "Stalker". Up-close, Stalkers are very aggressive and will charge immediately at the victim without hesitation. Stage 2 may take place anywhere between a few days after the start of the infection (Stage 1) and up to a year (Stage 3).
Their strength is equal to a fit human being as their attacks can interrupt actions made by the character.
Strategy
Some Stalkers have a bioluminescent vein in the face, and some of them are known to be impossible to punch, forcing the person to shoot them or use a melee weapon. This makes them deadly in combat especially when confronted in numbers. Therefore it is best to keep your distance when trying to confront a group of Stalkers. The character may lure Stalkers into one place then use explosives to kill them in numbers.
They can also be shived and strangled like Runners but the character must approach them with caution without getting seen or caught otherwise the Stalker will charge at the character nearby. They may also run away to take cover if they see a character approaching from a distance.
Clickers
Clickers are the third and most distinctly recognized stage of the Infected. They have had prolonged exposure to the fungus, and now possess strength that significantly surpasses the average human. This makes them deadlier, but at the price of being completely blind due to fungal infection overtaking their face. However, Clickers are able to maneuver through areas by utilizing echolocation, which produces noticeable clicking noises to locate sources of sound.
Clickers appear less human than Runners, with faces that are skewed and scarred by fungus developed from their infected brains. Clickers are dangerous and much more aggressive than Runners, and no longer resist the fungus because their humanity has been eliminated.
When provoked, a Clicker will immediately enter into a "berserk mode," aggressively flailing its arms. At this point, if a survivor has been "spotted" by a Clicker, the Clicker will attack head on and will not flinch when shot unless using high powered rounds.
Strategy
If the character is grappled by a Clicker, they can save their life if they have a knife. If not, Clickers will bite and rip into the neck, instantly killing the character. It is possible to avoid or distract Clickers instead of fighting them directly by throwing small objects away from you and your path. Boxing a Clicker with bare hands is ineffective. Most of the time the Clicker's berserk mode will override a punch.
Clickers can be stealthily killed by either using a shiv or a bow and arrow. Still, using an arrow to kill a Clicker requires a headshot; this has an 80% of chance of breaking the arrow. This is risky, however, as the fungus grows around their face and acts as a sort of armor, which significantly reduces the chances of killing them with one shot.
It is fairly easy to outrun a Clicker, as their crude echolocation has a short range. When far enough or behind cover, the Clicker will get confused and click in all directions. This makes them easy to avoid as you just simply have to crouch still behind cover roughly 10 feet away as the echolocations only range a few feet unless you fire a gun.
Bloaters
Bloaters are the fourth, final, and most dangerous stage of the Infected and are immensely strong. Bloaters take the longest to develop, making them the rarest of the infected. They are covered in thick fungus that effectively doubles as armor plate. Due to this protective covering, they can withstand multiple hits from weapons such as shotguns and hunting rifles, and even hatchets and machetes making them extremely tough to eliminate. Bloaters are extremely aggressive, but are rather slow-moving causing them to be more predictable than other infected. Like Clickers, Bloaters 'see' using echolocation to locate and trap a survivor as the fungus has completely deformed their face and blinded them, though their echolocation is a lot less refined than the Clickers. If a Bloater grabs a survivor, it will forcefully rip their jaw apart through brute strength, or grab the survivor's head and smash it, killing them instantly. There is no way to defend yourself if grabbed - the result is an instant kill.
Bloaters will throw sacks of Mycotoxin, a toxin produced by many fungi in the real world. The sack will explode on impact, spraying the target with the toxin. The Mycotoxin will hurt the player over time, so it is recommended to avoid these deadly clouds.
Bloaters, like all Infected, have a weakness to fire. When set ablaze a Bloater will wildly thrash around trying to put out the flames. After being immolated, they will be charred black and smoking. Their armor will also be charred and brittle, making them vulnerable to low-powered weapons. Despite this weakness, fire-based weapons such as flamethrowers or Molotov cocktails still require multiple uses before killing a bloater. Their other less effective weakness if fire is inaccessible is armor piercing rounds from a weapon like a hunting rifle.
Strategy
The most efficient way to kill bloaters is to throw a Molotov cocktail or use the flamethrower on them. Then, while they are stunned, use a shotgun and shoot two or three shots to kill the bloater, saving supplies and ammo.
Another very efficient way is to throw a Molotov cocktail then use a bomb while they are on fire.
Death and Corpse
Even after an infected individual has died, the fungus will continue to grow. By this point, the corpse becomes skeletal and brittle, and the fungus sticks the body to whatever surface it is on. The fungus will begin to emit spores from the body which will infect anyone that breathes them. To combat this, survivors who know they are at risk carry gas masks.