-If I were a very sick-minded ex-surgeon, I would have connected their blood-circuit. Without it, survival is impossible from the start. First: you can't survive eating excrement from just one person, second: as the urinary tracts are not connected either, the second and third subject would die from dehydration even before they'd die from hunger and third: connecting the rectum with the mouth makes it possible for bacteria from the lower gastric tract (f.e. E.Coli) to infect the upper gastric tract of the second person resulting in diarrhea in the first place, probably followed by generalized infection and death later on. There's a reason why the intestinal tract is segregated.
-And my last point is the following: the second girl dies -apparently- from generalized infection. If that's the case, the first girl would be evenly infected. You should also be aware that dying from generalized infection is quite a slow death involving very high fever and tissue necrosis, which she doesn't shows symptoms of.
Except from the medical point of view there are other illogic things: the police's behavior towards the doctor, the neighbors hearing screams(but not another house in sight, and the police shows up very late compared to when the screams might have been heard). And how the hell did the doctor managed to plant the scalpel in the first policeman's neck while unable to do anything else but crawling?