I'm looking through the handbook to see what majors I might be interested in, and some of them don't sound all that different from each other, like Micro+Immuno/Patho/Defense and Disease, and Physio/HS&F. How do you guys go about picking a major from these? 
You'll see the differences in your cores next year.
Doing Microbiology means you have to learn the characteristics of many bacteria and viruses (maybe some parasites too). You learn they cause disease, how they get transmitted, how to diagnose them, etc. You also learn how to treat them. Maybe some epidemioloy too.
If you did VCE bio then you might already sorta know what immunology is kinda like already. It's pretty much complementing microbiology - in micro, you learn how things invade you, while in immunology, you learn how your body repels it.
Pathology is looking at disease. Allergies, some inflammation, how things die, etc. Don't worry about Defense and Disease as this year is its last year, and its being cancelled. It was a pathology+immunology major.
Physiology is how the body functions - eg how you breathe, how your nerves work, how the circulatory system works, how your kidney works, etc.
Anatomy (HSF major) is pretty self-explanatory though - identify a structure, talk about why its signifciant, repeat.