The Spitter

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

She 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 stay in one spot for too long, quickly depleting the Survivors' health. When killed, acid will spew from The Spitter's body, making it impossible to cross the area around her with out taking damage. The Spitter may also be able to jump high distances, as seen in one of GameTrailers videos.

Extent of Mutations
Through mutation of the Infection the Spitter has developed the ability to project an extremely caustic acid from her mouth in a mortar-like fashion, projecting a concentrated ball of damage acid that explodes on contact and spreads into a large pool. This phlegm bomb has the ability to ignite gas cans and propane tanks and 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, signs that she may not be entirely resistant to the acid she is coughing up. Her proportions are awkward and stretched, including the elongated neck and bloated belly, which are most likely used to accommodate and produce the fluids. While running, a trail of acid can be seen dripping from the creature's mouth.

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. Still, the Spitter may have been developed from a stronger strain of the Boomer Infection, creating an entirely new Infected.
 * 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 rupture, causing the acidic goo to spill and pool where she was killed, along with her body.
 * 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 this X-ray is a person that has been infected with the same strain of the virus as the Spitter.
 * From the Left 4 Dead 2 Demo, Spitter goo apparently has a slight delay from when it hits the ground and when survivors start taking damage from standing in it.