The Boomer is a type of Special Infected from Left 4 Dead. The cause of this kind of mutation is unknown, but the result is a heavily-bloated Infected with large boils appearing in various places on the body. The Boomer produces an extraordinary amount of bile within its body which causes its stomach to distend to absurd proportions. In fact, the buildup of pressure within its digestive system is so great that the dermal and epidermal tissues around the abdomen have been stretched and thinned out almost to the point of rupturing. Any attack against a Boomer can rupture its stomach, causing the creature to explode violently (hence its name) and scattering blood and bile over a wide radius.
Boomers usually attack from a distance. Their main method of attack is to vomit up large amounts of bile and douse victims in it. In the case of normal humans, this would be enough to pass on the Infection, but has no such effect on the player characters who are immune. However, Boomer bile also has some sort of pheromonal effect and will attract swarms of Common Infected.