Dr. Marcus’ life’s work was the breeding of Leeches, which were infected with the T-virus. Marcus spent years breeding the leeches, watching their intelligence grow to the point where they had a collective intelligence and would even imitate their master. The leeches grew larger, developed an eye like structure and had enlarged mouths.

These leeches would attach themselves to a human, normally attacking in groups, and consume the flesh vigorously. They would also transfer the T-virus to their victim, essentially creating zombies in the process. The largest leech attack occurred on an Umbrella train heading to the training facility; the Queen Leech commanded his minions to kill everyone on board.