What if there was this disease that affected men and made them latch onto people they felt strong emotions for, be it friendship or animosity. Once they had done so, naturally the disease would spread to the other man who would do the same, probably to the the person they caught the disease from in the first place. If this disease was left untreated, then, over time, the victims would lose their own personalities and appearances. They would slowly morph into either a large eyed, childlike creature with a tendency to dress as a woman or a distended, taller and stronger but not much more masculine figure with an inability to discuss emotions. It starting to sound a little bit more like a parasite at this point, like that fungus that burrows into ants' heads and makes them go mad. At this point, the vestiges of the person that they once were hang off the victims like window dressing. Like zombies, the best thing to do to respect the person they once were and contain the disease is to shoot them in the head.

As a parasite, it sounds a bit like a Dr. Who monster, but I don't write that kind of fic.