He is one of three personal body guards to The Reservation's leader, Willem Clark. Measles is one of a very few ghouls who was conceived rather than transformed by radiation. At quick glance, Measles looks pretty much like any other ghoul; scabby, gray-green skin that is pealing away from the bone on certain parts, little to no hair, yellow eyes, etc. But upon closer examination, Measles' scabs, lesions, hair patches, and teeth gaps seem very uniformed, almost as if they were meant to be there and serve a function. Maybe this is the result of the breeding process, maybe Measles just got lucky, no one really knows for sure. But what is known is that Measles is a gifted hand-to-hand and melee fighter. He seems to know exactly where to hit someone to cause the most damage possible, which is quite unexpected from a gaunt ghoul. But this is one of Measles' greatest strengths, deception. Measles' unassuming posture and meek appearance makes all foes not think twice about him being a threat. However, by the time the shit hits the fan, the cocky enemy is laying face first in a puddle of his own blood and urine. Measles gets his strange name from his two large, bulbous-like tumors that stick out from his neck like a severe case of measles... well, that and his strange fascination with children's diseases. It's this fascination that also allowed him to be a fairly good scientist and pretty good at field medicine. He hates humans, but will go with the player if Willem orders him to, he is only hospitable to other ghouls, has a hidden agenda that differs from the PC's, and is willing to critically hit the PC in the back of his head, ghoul or not, to make sure his agenda sees the light.
Willem Clark is said to be the ghoul, "Messiah", leader of the Reservation, who wants to know about the other settlements in the wasteland; how big, how organized, and most importantly, are any of them a threat to the Reservation. Willem will order Measles to go with the Prisoner (if they will accept), under the guise of curiosity, exploration, and the hopes of establishing trade through a rail system. Once Measles feels he has learned enough (basically, after visiting Hoover Dam, New Canaan, and as a bonus, the Grand Canyon for its uranium mines), he will request to go back to the Reservation to give a report to Willem Clark.
In a nutshell, Willem has built a "dirty" bomb out of what uranium was left in the nuclear missile labs in the Reservation, and he wants to deliver, via rail and Nuclear Nellie, said dirty bomb to the community he perceives as his biggest threat. The bomb is meant to slow down the community's progress and hopefully dissipate the people after radiation sickness kicks in.
So, the second hub of Measles' mission is to establish a rail system, with the PC's help, between the Reservation and either Hoover Dam or New Canaan. Willem's preference on which to bomb is TBD. Possibly depending on how much the PC helped each community to better itself or screw it up. Willem will then bomb the "better off" community – i.e. the community most likely to prosper. Willem will then mount Nuclear Nellie on his makeshift train, park the railcar about thirty miles outside his enemy's location, and fire his dirty bomb out Nellie's massive barrel.
Lastly, if the PC has been through the Grand Canyon and discovered the uranium mines, Willem will want a rail system to go there so he can mine more uranium. His ultimate goal is to eliminate all possible threats, use the newly mined uranium to make a dirty bomb arsenal – or maybe even a couple of nuclear bombs if there's enough material – perfect the ghoul procreation process, and wipe out all the smooth skins so the "Children of the Wasteland" may rule the Earth.
Measles has not appeared nor is he mentioned in any published game. He was to appear only in Van Buren, the canceled Fallout 3 by Black Isle Studios. It is not certain if he will appear in any future Fallout game.