I don't remember much from undergrad microbiology, but I do recall the "bug" that I was assigned to present to the class... Aspergillus Fumigatus. It's everywhere, and it's mean, and it's getting better at resisting the azole drugs that it is treated with, and before azole drugs, most treatments were almost as deadly as the bug!!
Wikipedia entry (with cite): "it is estimated that A. fumigatus may be responsible for over 600,000 deaths annually with a mortality rate between 25 and 90%"