Can a drug cause infection? That depends. Chemical based infections aren't unheard of though, but in the realms of this fiction P30 was created from experimentation on Progenitor.
Again, there was no way to determine that at the time regarding Jill. There were clearly other means of B.O.W.s caused from more than just majini. Ruling out majini doesn't mean you automatically know someone can be saved or not.