hey fellow Dukie:
don't know who you are, but we may know each other since we both grad of 2002. anyhow, I took my step I last year and for Micro/immuno here is what I suggest. Appleton and Lange Immuno section is really good, but high yield immuno is just as good. I think if you are in a tight time schedule then do high yield (this will take you one full day). But if you are a relatively fast reader you can do AL immuno in about 1 and half days.
For micro. alot of people recommended made ridiculous simple, but it really depend on your style. I would recommend the micro cards, NOT THE BRS micro card, but the other set. It is really good, esp. with flow charts to help memorize and case descriptions. I liked them the best.
I did not remember seeing a single question on Fungus and parasites...so if you are on an info overload, then leave the fungus and parasites later. Bacteria and viruses are covered the most. And most of the questions ask what is the most probable organism. good luck!