The Infected.

The Common infected
   

(some reference taken from Left4Dead’s wiki and The Last of Us’ wiki)

Approximately three years ago, a mutated strain of the fungus Ophiocordyceps Unilateralis appeared within the United States and began to rapidly spread throughout the nation. Within months, roughly 65% of humanity had either been infected or killed off by what is scientifically referred to as the Cordyceps Brain Infection.

According to old newspaper scraps salvaged from the ruins of press buildings, the fungus had originated in South American crops. From what’s been observed by both research facilities and survivors, once the spores reach the human brain, it begins growing mycelium within the brain tissue and killing off brain cells. This causes erasure of memories and insanity, modifying the host’s instincts to seek no goal other than spreading the fungus. Eventually, the cordyceps kill its host, and then grow out of its head to continue to spread its spores.

Common infected go through four stages once exposed to Ophiocordyceps Unilateralis: runners, stalkers, clickers, and bloaters. Abnormals deviate between the ‘runner’ and ‘stalker’ stages- though those’ll be discussed later.

  “Runners are the first and weakest stage of the Infected who have just 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.” (Fearon, William C. “On the Cordyceps Brain Infection.” Keeping Humanity From Extinction 68.3 (3048):577 JSTOR)

 

Runners are the most frequently encountered of all the infected, and the easiest to defeat. They seem incapable of feeling any pain, but they are just as easy to kill as any normal human- easier, almost. Their strength lies in their numbers, which are commonly referred to as “hordes” when in clusters of twenty or more; though powerful, they are by no means invincible. Extremely sensitive to loud noises and movement, Runners can easily be distracted by things such as car alarms and the bright lights of flashlights.

 

“Stalkers have the vision and speed of Runners and are no longer bound with human conscience, bearing the ferocity 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 with only one eye left, 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. This stage may take place anywhere between a week after the start of the infection and up to a year.” (Fearon, William C. “On the Cordyceps Brain Infection.” Keeping Humanity From Extinction 68.3 (3048):577 JSTOR)

Almost as frequently encountered as Runners, Stalkers are usually clustered in groups of three or four when not alone. They can be seen easily in dark settings due to the bioluminescent fungal vein growing from their eye. They are highly sensitive to their surroundings, as the spore growth from their head is able to pick up any sound and movement from a five meter radius by way of pseudo-echolocation. They are not as easily distracted as objects such as Runners and also seem incapable of feeling pain- some have been recorded to have the strength of a fit human being.

 

“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 and Stalkers, with faces that are completely skewed and scarred by fungus developed from their infected brains. The fungal plates have apparently adapted to keep the host/pathogen alive and able to spread, as a direct shot to the face with a powerful weapon such as a revolver will more often than not just break off a chunk of the mold, leaving the Clicker's brain unharmed. Clickers are dangerous and much more aggressive than Runners, and like Stalkers; they no longer resist the fungus because their humanity has been completely eliminated. They also seem to have enhanced strength capabilities. Clickers also possess what seems like very high intelligence, never leaving an area when they have heard food run by them, and will constantly check every available nook and cranny to locate the source of the sound with little regard to give up on the task at hand. Interestingly, most of the Clickers seen in the game appear to have been female humans. It can take the infected up to 8 years to reach Stage 3 (the soonest transformation recorded was 3.4 years).

 

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.” (Fearon, William C. “On the Cordyceps Brain Infection.” Keeping Humanity From Extinction 68.3 (3048):577 JSTOR)

Aside from the final stage of infection, Clickers have been recorded to be the most vicious of the infected: upon having a survivor in their grasp, they will proceed to bite and rip into their neck. Though persistent in their searching, they are not as highly sensitive to their surroundings as Stalkers are. The armored plating around their head prevents anything less than a high-powered military-grade weapon from delivering a deathblow by headshot, although the chest and throat area has been proven to be just as easily targeted. It is quite easy to outrun a single Clicker, as unlike Runners and Stalkers they merely jog at a moderate pace, although a cluster of them may prove a problem. Clickers have been proven to be easily susceptible to molotov cocktails, as they will be drawn by the noise and walk straight into the flames, burning themselves to death.

“Bloaters are the fourth, final and most dangerous stage of the Infected. 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 the Shotgun and Hunting Rifle, 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. Because the fungus has completely deformed their face and blinded them, 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 survivors over time, so it is recommended to avoid these deadly clouds.” (Fearon, William C. “On the Cordyceps Brain Infection.” Keeping Humanity From Extinction 68.3 (3048):577 JSTOR)

 

Like all infected, Bloaters have a weakness to fire- when lit up, they will thrash around wildly, slamming into walls and attempting to throw themselves in the direction of the nearest survivor. After the fire wears off, their plated fungal armor will be charred, black, smoking- and not to mention considerably weaker. Once their armor has been burnt, guns will have more effect on the Bloater, though it will still take a considerable amount of ammo to do so. It is advised to aim for the Mycotoxin patches around the Bloater’s body first- on the lower right abdomen, upper right chest, and upper right back- to completely disable its ranged attack. Do not ever attempt to charge a Bloater with a close ranged weapon unless attempting to suicide bomb it.

 

Now- abnormals, so named due to the fact that they don’t follow through the regular four stages of the common infected. Between the first and second stages of the Cordyceps infection, an infected has a slim chance of mutating and deviating away from the normal stages of infection, becoming instead a much more ‘powerful’ infected with inhuman abilities that they have been named after. Without the proper caution, it is very easy for an abnormal to take down the unsuspecting survivor.