The Erlenmeyer Flask

When Deep Throat points out a news story about a fugitive who apparently drowned, Mulder and Scully cannot see what makes the case special but they follow it up anyway. With Deep Throat's insistence, they discover evidence of secret government project code-named Purity Control, which uses human test subjects and infects them with extraterrestrial DNA. However, the evidence and everyone who has seen it is quickly being eliminated.