(Capcom's primary storyline)
The Giant Leech was a leech infected with the t-Virus that took up residence within the waterways beneath the Raccoon General Hospital. Apparently different from the leeches James Marcus was working with, this leech was, like its smaller kin, extremely sensitive to fire. During the Raccoon City Destruction Incident, a group of survivors attempted to escape the hospital by using the sewer, unaware of the giant leech's presence. They accidently rammed it with a boat, causing it to become enraged and attack. It was killed during the fight.
Attacks and strategy
The giant leech is one of the most passive bosses from all the series, and will only attack close objectives. If you move too far away from this creature, it will lose interest in you and go the other way. It's recommended to shoot at it enough times, or lure into the three ceiling valves before the pipe, as each of them will cause 300HP worth of damage to it. Usually the Leech will lose interest in its prey and stay idle, searching with its tentacles which it may also use to attack the player. It's recommended to stay in a middle range away from it, due to its headbutt attack that will inflict heavy damage to the player (450HP for the most in HARD), and the poison that it spits at the player, which doesn't do much damage but has a 20% chance of inflicting POISON status. Most of the time, when shooting the pipe, it will inflict 2000HP on it, but if the Leech persists in living, lead it through the next tunnel section and shoot at more of the ceiling valves. If you manage to lead the leech into where the boat was docked, it can then be beaten to death by stomps (if you are up to the NO WEAPON BONUS), or you can examine and then shoot the Gas Can, which will do 9000HP on it, killing it instantly on any difficulty but VERY HARD.