The Spitter

The Spitter is a Special Infected that made her debut in Left 4 Dead 2.

It makes a hacking sound before spitting out what appears to be a ball of mutated stomach acid, which, upon hitting the ground, spreads out into a large pool of murky green acid. Any Survivors within the area receive massive quantities of damage over time. This makes it exceedingly difficult to camp in one place for too long, otherwise the Survivors' health will quickly be depleted. It can be assumed the substance it shoots from its mouth has corrosive properties, although it appears unable to pass through obstructions, even bouncing off hard surfaces such as walls. This brings to mind the question of whether or not the Infected could effectively spit at a wall from an angle and divert its acid to a camping group around the corner. This would cause a major disadvantage for Survivors camping out on Versus mode. She apparently also possesses the ability to jump high distances, as seen in one of GameTrailers videos. In that video, she is seen jumping to the height of a roof, spitting the acid at the Survivors, and then falling down, however, it is unknown if she still possesses this ability since the Spitter got a model change and was made fatter.

Extent of Mutations
The Spitter is the second female Special Infected to appear within the Left 4 Dead franchise. Through mutation of the Infection, it has developed the ability to project an extremely caustic acid from its mouth in a mortar-like fashion in that it is projected in a concentrated ball and on contact it spreads into a huge puddle of acid. This phlegm has the ability to ignite gas cans, and propane tanks which will likely be an advantage in Versus games. The Spitter's lower jaw, cheeks, nose and lips seem to be either gone or noticeably eroded. This shows that it may not be entirely resistant to the acid it coughs up. The creature's body proportions are awkwardly stretched, including the elongated neck, and the belly seems bloated, most likely to accommodate and produce the fluids. It also appears that the creature's stomach glows an eerie green, showing that this is where the acid is perhaps stored. While running, a trail of acid can be dripping from the creature's mouth.

Oddly, the Spitter has control over her widened maw, which should be impossible due to the burning away of her previous human mouth and surrounding muscles.

Trivia

 * It was speculated that the Spitter was a female variant of the Boomer, where the Boomer virus had had a different effect, resulting in the Spitter's damage-causing attack, yet at the same time replacing the Infected attracting hormones. This has been disproved as there are now videos of female Boomers. However, the Spitter may have been developed from a stronger strain of the Boomer Infection, creating an entirely new Infected.
 * According to recent gameplay videos from the PAX, the Spitter's goo no longer turns red, instead retaining the bright green color constantly. It is also noted that the Spitter's stomach appears to glow with the green goo.
 * According to recent gameplay footage, as well as the second Left 4 Dead 2 trailer, the newest Spitter model is the permanent one. She was made larger, sports a thin white bra and pink thong under her pants, and leaves a green slime trail from her mouth as she walks. Her legs no longer seem lengthened, but her neck is still extended. Her face is mostly deteriorated away, save for her teeth and most interior mouth tissue. When killed, she will "melt" and leave a small pond of the acid she spits (the body, however, does not actually disappear).
 * The Spitter is currently the first female Special Infected to be playable. Followed by the female Boomer
 * The Spitter's goo does not seem to harm other Infected, as they are most likely immune to its apparent acidity. This could be very useful for Smokers, Hunters, or even Boomers (there's nothing much more problematic for a survivor to be blinded, standing in acid, being attacked by a horde, and maybe even pounced by a hunter at the same time). However, she does not seem to have complete immunity, as there are large, irritated burns where her acid seems to have dripped on. Her mouth and face are also extremely twisted and appear to have melted, most likely due to spitting so much acid.
 * The Spitter appears to have an extreme version of the disease known as Acid Reflux. This is another reference to Special Infected possibly gaining their powers through bodily properties they had before they turned.
 * It should be noted that in Mercy Hospital, one of X-rays shows an elongated neck. It is possible that that X-ray is a person that has been infected with the same strain of the virus as the Spitter.