Perhaps this may seem morbid to you. But I'm fascinated by the bacteria, viruses and parasites which prey on humans, and how human behavior exposes, spreads, and (rarely) contains them.
Disease, epidemics, biowarfare, modern genetics, drug-resistance. Microbes have shaped all of human history and will continue to do so for the future to come. And it's a fascinating study, from the course of a given disease in a single host body, to the spread of an infection in a population, to human reaction and behaviors, to the personal tales of those who study and combat these tiny warriors.
And I have to take a moment here to thank my undergrad "Epidemics in History" teacher, who sparked this interest in me. I had never met a person who could wax lyrical over a description of stage 3 syphilis or expound the sociological reactions to the Black Death with quite so much passion.
Promed Mail - A medical professional's watch-list of outbreaks.
The CDC - The US's Center for Disease Control.
There are a lot of very good books out there written at the level of the general populace, if you're curious for a taste of what I mean. Have a look in my books page for a reading list!