Modified FEV

Background
The Modified FEV is a heavily modified sub-strain of the Forced Evolutionary Virus that was developed by John Henry Eden. It is said to be based on or related to a progenitor, the FEV Curling-13 sub-strain the Enclave's Chemical Corps had originally synthesized and tested in 2242.

Some changes include it's intended design of being compatibly inserted into a water purification system's schematics, such as the one found at the Jefferson Memorial. Also, highly unlike the FEV Curling-13, it does not target those that are pure humans.

Omega
According to President Eden, it will indiscriminately target and exterminate super mutants, ghouls, any and all other mutated creatures, and generally anything that has been at all effected by Wasteland mutation upon ingestion. This would include any Wastelander that had been originally born in the Wastelands and/or heavily exposed to it's background radiation and other mutational hazards, or hosts any other sort of mutational side effect at all as a result.  

Theoretically, the only people who would prove immune to infection are dangerous radiation level/mutation free pure humans, those that are born inside Vault-Tec Vaults or those born inside other similarly sealed environments and as a result, uneffected by Wasteland mutation. This is proven to be accurate in one of the ending slides if the player completes Project Impurity.  

In the add-on Broken Steel, if the player chose to contaminate the Purifier with the modified FEV, the medical facility of nearly every major settlement in the Capital Wasteland (except Tenpenny Tower and Paradise Falls) has at least two or three newly arrived sick patients lying on the beds, dying from modified FEV infection.

The patients hold their stomachs and moan that "it burns on the inside." If/when the player speaks with one of the doctors in a major settlement, they state that the're fairly certain that 'the condition' is not contagious to their knowledge.  

Doctor Preston does state however that they are burning the corpses of the infected dead just to be safe, out of fearful precaution that drinking the water might indeed transmit or otherwise result in a contagious plague of some kind developing as a result.  

And in Megaton, Doc Church postulates: "it'd be damned ironic if we were so used to the crap we had been drinking that actual pure water was killing us." '''

Related quests

 * The American Dream
 * Project Impurity
 * Take it Back!