Demand for medical illustrations is declining. People use digital cameras now.
After taking basic lvls of chemistry, physics, and biology I took a class in evolution and morphology. (Kinda botany and zoology combined into 2 quarters of a semester) A class like that which gave an overview of our natural world sounds like what your looking for. Alittle after that you have to study things more specificly, like acouple people mentioned. Microbiology, botany, Zo, virus study, ect.