Oh, I love this question!! Here goes:
Microbiology, biochemistry, physiology, nutrition, vitamin therapy and a slew of other things.
Microbiology, biochemistry and physiology have to be my favorites. I love the detail involved and all the intricacies. These three subjects have proven to be the only ones that can capture my attention for hours at a time. I find them absolutely fascinating. In fact, I'd much prefer to read about microbiology than read a novel or magazine! It actually relaxes me.