The Infected

"They might still look like people, but that person is not in there anymore."

- Ellie to Sam regarding the Infected.

The Infected are humans who have been infected and subsequently mutated into horrific new forms by the Cordyceps Brain Infection. The resulting creatures are highly aggressive, and will attack any uninfected human on sight.

The Infected are one of the main enemy types in The Last of Us, Left Behind, and The Last of Us Part II, the other being the human enemies.

Overview
The Infected are the result of a sudden outbreak of a mutant Cordyceps fungus that now affects human beings as opposed to merely insects. The fungus reportedly originated in South American crops; various newspapers in the prologue support this claim. The fungus infects the human brain, growing mycelium inside the brain tissue and killing the brain's cells. This erases their memories and drives them insane, modifying their instincts to seek no goal other than spreading the spores to others. Eventually the fungus kills its host, and will then grow out of the host's body to spread spores.

Public reaction to the Infected has been catastrophic. As the vicious fungus has taken hold of the majority of the population, the worldwide pandemic causes the total breakdown and destruction of society through widespread violence; the failure of martial law to contain the outbreak, the military's increasingly violent efforts to censor information, and the near-extinction of humankind. The Fireflies – a militia group formed in response to military oppression – have conducted a mostly futile war against the barely-functional government. They believe they can create a vaccine through study of the fungus and its hosts, one of the last organizations to do so.

Those who are clear of infection are ordered by martial law to reside within a Quarantine Zone. The military at one point bombed areas outside the Quarantine Zones to kill the infected and executes anyone suspected of infection on sight.

Infection
"[...] the antigenic titers of the patient's Cordyceps remain high in both the serum and the cerebrospinal fluid. Blood cultures taken from the patient rapidly grow Cordyceps in fungal-media in the lab..."

- Surgeon's recording For humans that have been exposed to a mutated strain of Ophiocordyceps Unilateralis, the fungus attacks the brain and begins to alter the behavior of its host. The host will then go through four different stages of infection: Runner, Stalker, Clicker, and Bloater; each stage is more dangerous than the last. However some have had resistance to the infection, notably Ellie; she hasn't turned despite being infected for a year; though her rash from the bite mark has worsened slightly.



The infection also spreads directly through contact with an Infected or via their corpse - that is, the infectious spores created by the Cordyceps and released from the host's body upon death. Spore development is the last stage in the life cycle of the Infected. This lifeless carcass serves as a breeding ground for the fungus to spread throughout the environment. When near death, an Infected will find dark, moist corners to die in. While this may be the end for the host, the fungus continues to grow. The Last of Us game team has stated that the people in the Quarantine Zones are relatively safe because they are in places with significant airflow, leading to a low spore concentration, explaining why the only places with significant spore toxicity tend to be underground or abandoned buildings.

In order for the fungus to infect a host body, the host must first be alive. Dead victims who come into contact with the infection do not undergo transformation. According to a pamphlet released by the Center for Disease Control, the victim can only become infected by inhaling the spores directly or coming into contact with an infected's bodily fluids, usually from a bite.

Infected sometimes do not go on to the next stage of infection as presented in the large number of Runners in Bill's Town.

Due to growing a fungus within themselves, all of the infected are extremely vulnerable to fire.

Physical appearance
The Infected humans have since undergone a strange mutation by a specialized strain of cordyceps. Over time, growths protrude out of the orifices of the human hosts and it changes their physical appearance. Their teeth are horribly rotten and jagged. The fungus on Stalkers, Clickers, and Bloaters is bio-luminescent. The fungus also seems to offer the host some protection against the elements, such as the cold. At later stages, their body begins to bulge, and more fungal growths can be seen on their skin. When the host is killed or dies from advanced fungal growth, the fungus continues to grow throughout the body, eventually emitting spores.

Behavior
"You know, as bad as those things are, at least they're predictable. It's the normal people that scare me."

- Bill Upon becoming Infected, the victims gradually lose any ability to think freely and begin to behave irrationally as the fungus spreads in the brain. Relying on warped instincts, the Infected will attempt to attack and/or eat any human they find. This becomes most prominent at stages 3 and 4, when their human characteristics begin to cease to exist entirely (e.g. feelings, thoughts, etc.). They lapse into their primitive, animalistic instincts, attacking anyone or anything that disturbs them, almost as a way to protect their territory. According to the Head Surgeon, fungal-growth in the limbic regions normally accompanies the prodrome of aggression in infected patients.

The fungus spreads to the host's eyes as it grows, covering and blinding them. They then develop a primitive form of echolocation to locate prey, which results in them making a distinctive clicking sound. After some time, the fungus kills the damaged host, usually in a closed-off space so the fungus can spread further. The Infected, while having a higher pain tolerance than ordinary humans, are still susceptible to conventional injuries. A Runner, when not attacking survivors, will moan in pain due to the fungal growth deforming its body and Clickers will occasionally struggle while being shivved. Sometimes Infected that still have their entire face intact look sad or worried as they attack, indicating that first and second stage Infected are aware of what they are doing, but cannot control themselves. Gunshots are capable of startling Runners and Stalkers, so they seemingly feel fear in the early stages of infection. The Infected have also developed very sensitive hearing, as they can hear sounds that normal survivors cannot, such as the player hurdling nearby obstacles.

Runners


Runners are the first and weakest stage of the Infected who have just recently entered their transformation. People who have been infected will enter this stage within 1-2 days with increased irritability and hostility towards others being a key symptom. Since the eyes are first targeted by the fungus, Runners have poor eyesight, but maintain some human characteristics. For example, their moans are still human, just like their overall appearance. A way to tell them apart from normal survivors is their glowing orange eyes. They also stay in a hunched position when motionless or when moving around passively, unlike normal survivors. Their body language is sluggish while they are passive, suggesting they are resisting the fungus's influence, albeit usually in vain. 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. They will also become aggressive if a non-infected individual is extremely close in front of them. 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 in attempts to trample the survivor.

Like survivors, they can be strangled when grabbed from behind.

Runners seem to have an easier time than humans when it comes to spotting Joel and Ellie as, with humans, Joel can just walk up behind them and strangle them; something that is not possible to do with Runners without them spotting Joel. Also, moving Runners seem to be capable of spotting Joel when crouch-walking, indicating heightened awareness. The Runners can also grab Joel and will try to bite and infect him while doing so, but they are less likely to be successful due to the fact they can be shaken off, unlike the physically enhanced Clickers.

Strategy
Anything works against Runners, and generally, the main concern when fighting them is the possibility of being swarmed by them in large numbers. In which case, one may consider attempting to escape their detection by sprinting away to a distance and diving behind cover. Stealth is a very good option while dealing with them, although not entirely necessary. As with any other choke-able enemies, the player needs to be careful about being spotted by other nearby enemies while choking them out one by one. Attempting to strangle Runners from the front will cause them to become aggressive and alert nearby Infected, so it's advised to avoid such. Individual Runners are also very easy to melee even with bare fists, so using up any ammunition when fighting them can be wasteful when there are only a few of them. There is little gained, but plenty lost if Joel uses a shiv on them, as shivs are best saved for Clickers, shiv doors, and Stalkers to obtain the The Hunter Becomes the Hunted trophy.

The Runners are at such a weak stage that most survivors can easily defeat them with little trouble, such as the Cannibals at the Colorado Mountain Plaza; they could easily out brawl them in melee combat and could flip them over their heads and stomp them. Others however, like the average Smugglers, seem to struggle, being overwhelmed by them in seconds.

Also, Runners are extremely dangerous around Clickers and Bloaters as the former are still able to still see survivors, alerting the latter to their presence. It is best to strangle any Runner in the vicinity, as this will maintain the quiet ambiance needed to bypass Clickers and Bloaters.

Stalkers
Stalkers are the second stage of the infection. They have the vision and speed of Runners, with the ferocity of Clickers. The most notable physical traits that define the Stalkers are 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 directly at the victim. Stage 2 may take place anywhere between a week to a month after the start of the infection (Stage 1) and last up to a year (Stage 3).

Their strength is equal to a fit human being, as their attacks can interrupt actions made by the player. Although their grab attack can be shaken off, it will still result in an instant death from blood loss/infection if the Stalker succeeds with the attack.

According to the Stages of Infection collectible, the modus operandi of a Stalker is to "hide and ambush victims." The collectible implores readers to check their surroundings, but no other information is provided. Their croaking may be heard from a distance in various areas, but it only serves as ambiance rather than a threat.

Stalkers are rather rare and are only encountered as the main enemy twice: once in the basement of the hotel in Pittsburgh with a Bloater and once in the Sewers with a pair of Clickers. They are also encountered in the Left Behind DLC by Ellie for the first time on their own. Strangely, all of these places are either flooded with or located near water.

Strategy
Some Stalkers have a bioluminescent vein in the face which makes them easier to see in the dark. They are fast and deadly in combat, especially when confronted in numbers. Because of this, it is best for Joel to keep his distance when trying to confront a group of Stalkers. The player may lure nearby Stalkers and then use explosives as they approach Joel.

Stealth is an option, but not advised due to them being almost impossible to grab while moving because of their heightened awareness. They may also run away to take cover if they see the player approaching from a distance. There is a tag worth 100 in-game currency called The Hunter Becomes the Hunted for stealth killing a Stalker from behind for the first time though this is not advised when there is a group of them because there is a high chance they will overwhelm the player.

Because they are stronger than Runners, Stalkers can override Joel's punches by grappling or striking him. One way to combat this is to throw a brick or bottle at them then attack, or simply use a melee weapon rather than fight barehanded. However, because they are often encountered in large numbers, it is best to rely on ones armed fire. One blast from a shotgun or El Diablo is enough to take them down, as is a headshot from the hunting rifle.

Unlike the more able bodied Joel, it is challenging for Ellie to combat them, as she only has her pistol when facing them. Because of this, it is best for Ellie to rely on bricks in the area, throwing them directly at the Stalkers to stun them so she can stab them with her switchblade.

Clickers
Clickers are the third and most iconic stage of the Infected and take roughly a year to reach this stage of infection. 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 taking over their face. However, Clickers are able to maneuver through areas by utilizing echolocation, which produces noticeable clicking/screeching noises to locate sources of sound, hence their name. Like Runners, Clickers can sometimes be found in a passive state.

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 but exposing the victim's face and rendering them able to see once again (until the fungus regrows). 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 seem to have enhanced strength, as they can overwhelm and kill Joel if grappled and always succeed if attacking other humans, like the group at the mountain plaza. Clickers also possess what seems like very high intelligence, never leaving an area when they have heard prey 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.

When provoked, a Clicker will immediately enter into a "berserk mode," aggressively flailing its arms and 'roaring'. 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 the player is using high-powered rounds.

Strategy
Punching a Clicker with bare hands is ineffective — after three punches, Joel will not have time to back away and will be instantly grappled by the Clicker, especially if the Clicker is in berserk mode, where it overrides Joel's punch. It is recommended that players use a melee weapon if they choose this form of attack. If not, the ideal weapons to use are the shotgun, El Diablo and the flamethrower because they, when used effectively, guarantee kill.

If Joel is grappled by a Clicker, he can save his life if he has a shiv and at least level 1 of the Shiv Master skill, unless the player is playing on easy mode, where they already have the skill. If not, Clickers will bite Joel, resulting in a game over. It is possible to avoid or distract Clickers instead of fighting them directly by throwing bottles or bricks.

When the Clicker does manage to go into berserk mode, it is advised to keep one's distance before aiming at it or trying to hit it to avoid being grappled.

If the player desires to engage them stealthily, Clickers can be killed by either using a shiv or the bow and arrow. However, using an arrow to kill a Clicker requires a headshot - which has an 80% chance of breaking the arrow - or two chest shots, which will alert them, so unless done fast enough, the player will be detected. This is risky, however, as the fungus around their face acts as a sort of armor, which significantly increases the chance of the arrows breaking. Shivs are also needed to unlock Shiv Doors, which contain special items that are needed if one wants to have the best chance of surviving. However, due to Clickers being blind, they player can safely shiv them from the front, provided they are in their passive state. Clickers that are active can only be shivved safely while the player is behind them, although the having Shiv Expert unlocked allows a player to shiv them if grappled.

It is fairly easy to outrun a Clicker, as their crude echolocation has a short range and, unlike Runners or Stalkers, they move at a moderate pace. The Cannibals realized this, avoiding guaranteed defeat in close combat and attacking with firearms from a distance, where they were safer. When far enough away or behind cover, the Clicker will get confused and click in all directions. This makes them easy to avoid as the player simply has to crouch behind cover. The echolocation only ranges a few feet, unless the player fires a gun.

If stunned with a brick first, players can kill Clickers with melee attacks. If done successfully, the player can sprint towards them and finish them off with a running melee strike (a running punch combo can also work, but is more risky), This method could be attempted approaching the Clicker from the front, though the back is usually the safer option.

Clickers are extremely susceptible to Molotov Cocktails and groups of Clickers can be taken down by them. When encountering multiple Clickers, simply throw a Molotov into the middle of the area. Upon hearing the sound of the bottle breaking, all Clickers will bee-line for the noise and, in the process, walk right into the flames, burning themselves to death.

Bloaters


Bloaters are the fourth, final, rarest, and most dangerous stage of infection as they take the longest to develop, taking years to reach this stage. They are covered in thick fungus that effectively acts as armor. Because of 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, making them 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 violently rip their jaw apart through brute strength, or grab the player's head and smash it, killing them instantly. There is no way for the player to defend themselves 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, are vulnerable 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 is armor piercing rounds from an upgraded weapon, like the Hunting Rifle. It will take fewer shots to kill if the weapon has the Power and Armor-piercing upgrades. Considering the rate at which Cordyceps grows after its fruiting body has emerged from the hosts head, the infected is estimated to take from 10 to 15 years to reach stage 4.

Bloaters are found in the five following locations:
 * 1 in the gym of the High School in Lincoln. (Must be fought)
 * 1 in the basement of The Hotel Grand in Pittsburgh (Can be avoided, not there on easy mode)
 * 1 in the dormitories in the University of Eastern Colorado. (Can be avoided; drops a Firefly Pendant if killed)
 * 1 in the mill during winter at the Lakeside Resort. (Must be fought)
 * 3 in the underground tunnel in Salt Lake City. (Can be avoided)

Strategy
The most efficient way to kill bloaters is to throw a Molotov Cocktail or use the Flamethrower on them, due to their vulnerability to fire. When they are stunned, use 2-3 rounds from a shotgun to kill the Bloater to save supplies and ammo. However, a burning and thrashing Bloater can still grab Joel and kill him, so under no circumstances should one ever let a Bloater get within melee range. To counter this, two shots from a fully upgraded El Diablo are also enough to kill a Bloater.

Another efficient way to kill Bloaters is to throw a Molotov Cocktail, then use a nail bomb while they are on fire. Do not get too close, however, as it will always charge at the player.

The Bloater actually draws its Mycotoxin bombs from bright, deformed patches growing on its body. Each Bloater has three: one on its lower right abdomen, one on its upper left chest, and one on its upper right back. Shooting any of these growths destroys them and reduces the damage required to kill the Bloater. Destroying all three will completely disable the Bloater's ranged attack, limiting it to charging at the player. This significantly increases the player's options for dealing with the Bloater, as it can no longer harm the player from a distance and is easily lured into traps. If unable to destroy them, the Bloater will take a short amount of time to throw the sack of fungus; when they then raise their arms, run away from the direction the Bloater is throwing the sack.

When dealing with a Runner, Stalker or Clicker, if the player has access to a brick or empty bottle, as well as a blunt weapon, the player can stun the Infected by throwing the object at it, and then continue by running towards the Infected while swinging a melee weapon. This will result in an instant kill. However this is ineffective against a Bloater, as its hardened face is impervious to thrown bricks/bottles and standing still may invite it to lob a fungus bomb at the player or close in for the kill.

For the early game, when the player does not have the Flamethrower, they can use their shotgun. Running in different directions, stopping and then firing one shot is time consuming, but it will work after many shots. If they run out of ammo for the shotgun, they can quickly switch to the hunting rifle. This is also viable on the higher difficulties, as one is better off avoiding enemies altogether to conserve your ammunition or when they have run out of Molotov Cocktails and Nail Bombs. Even though Bloaters move slowly, they can still charge when they know the player is near so it is advised to avoid becoming cornered when avoiding a Bloater's grapple; special awareness is key to avoid being trapped.

Death and corpse
Even after an infected individual has died, the fungus will continue to grow. It grows into big fungal plants around the body so much as only a few inches but can cover large areas. 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, most survivors carry gas masks, although if the person is immune to the infection they have no need for one.

They are commonly found in places that are underground or wet such as sewers and basements.

Trivia

 * Interestingly, in-game Clickers jog when chasing Joel or an AI. However, in the cinematics I need a car and Name's David, Clickers do sprint.
 * Clickers also sprinted in the PGW trailer for The Last of Us Part II as well.
 * Clickers can be observed clawing and scratching the fungal growth on their eyes as if trying to get rid of their blindness.
 * In Techland's video game Dying Light, there is an Easter egg in which a zombie with its head shaped like a Clicker comes out of an elevator. In addition, it will drop an item called "Clicker" when killed.
 * In the cartoon The Amazing World of Gumball, the episode "The Parasite" features a Clicker when Gumball merges with Darwin in order to warn Anais. He also mentions that he takes the zombie's form from a video game.
 * The Infected appear to rest standing up. They do not click or attack even when the player is directly in front of them, unless provoked, while resting.
 * If the Bloaters in the Pittsburgh hotel basement and UEC dormitory kill Joel, they will actually knock the mask off of his face in a special death animation.
 * When controlling Ellie, the Bloater death scene changes; the Bloater simply punches Ellie, killing her with its sheer strength.
 * In-game materials, concept art, and the "Grounded" Making Of documentary all show that the fungus is able to rip through bone.
 * The skin of Clickers visibly "chalks," or forms large patches of scale-like tissue that glow in the dark. The patches are seemingly an earlier stage of the fungal infection growing out of their hosts' bodies, before said growth engulfs the entire body as seen with the final Bloater stage of infection.
 * Runners do not moan as much nor in the same way as in previous beta demos as if in horrific pain but remain silent for the most part. However, they will screech when startled.
 * Despite Clickers using echolocation, their echolocation is very crude compared to creatures like bats where they will not notice that something is alive or moving unless it is directly in front of them. This supports the theory of being where the clicks come out, as their face plates are shaped like a, directing it straight ahead of them, but not all around.
 * Clickers do not click the same way as in previous beta demos where it would sound more like constant tapping of the tongue. In the final game Clickers click by waves of croaking rather than constant tapping.
 * Stalkers appear the least amount of times when compared with other types of Infected, with the player encountering them a mere three times in the entire franchise so far, twice in the main game, and once in the Left Behind dlc.
 * Even though objects can be thrown to distract the Infected, it should be noted that a major drawback to doing so is that it will draw them out of their scripted paths and allow them to freely roam the area, making it more difficult to predict their positions. If necessary, it is advised to have a steady flow of items to throw to keep them in the same area.
 * Some of the concept art in The Art of The Last of Us shows Runners wielding tools such as shovels, knives, and rakes.
 * Early concept art for the Infected showed them with an almost alien appearance, with insect like claws and glowing eyes, but the developers choose to maintain a human resemblance so it would make them feel more unnerving.
 * Unlike with hostile humans, flashlights do not alert the Infected to Joel's or Ellie's presence, although when shining the light in their eyes, they do appear to react to it by holding up their hands to shield said eyes. This could be due to their low intelligence, as it seemingly suggests that the Infected are too simple-minded to distinguish flashlights from any other ambient light sources they encounter.
 * A Clicker appears near the end of PlayStation's Greatness Awaits trailer.
 * In the University Laboratory, an X-ray of an Infected (possibly a Stalker) displays the fungal growth on the head.
 * As was revealed by Naughty Dog at PAX Prime Panel 2013 Event, there were originally supposed to be other Infected animals, including elephants, which are described as having fungal growths on their legs and mushrooms in their heads. However this design did not make it into the final game, therefore, it is unknown if in-universe animals are capable of becoming infected, as they often seem to be scared away by Infected. It's shown during The University chapter that monkeys can be infected.
 * Joel can punch a Clicker to kill them when the Clicker grabs an AI partner, such as Bill.
 * Bloaters are the only stage of Infected that do not appear in Left Behind.
 * The infected skeletal system appears to be much weaker than a normal human. Joel can stomp on a Runner's head or smash them against a wall, killing them.

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