The drop out rate for med school is under 5%. Infectious disease is a 3 year fellowship that's done after completing a 3 year internal medicine residency. Oddly, the average salary for infectious disease specialists is lower than that of internal medicine physicians. Economically, it doesn't make sense for internal medicine physicians to spend 3 years doing a fellowship only to make less money on the other end. On top of that, these physicians are missing out on an attending's salary during those 3 years of fellowship.