Queen Leech

When Dr. James Marcus was assassinated in 1988, one of the Leeches the scientist was working entered into his masters mouth and resided in his dead body. Over the course of 10 years, the T-Virus fused with the DNA of the co-founder of Umbrella, until the Queen Leech was ready to reap her master's revenge. Typically assimilating the young care-free Marcus, the Queen Leech eventually discards the body before revealing its true humanoid form.