"What's a pathogen?"
Reading about pathogens, as all be doing a bit of touch up to some areas of http://www.pathogenomics.bc.ca.
Bacteria is like little bugs. Real small bugs. There are three kinds of these "mini-bugs": Bacteria, Archaea and Eukarya. Bacteria are the simplest. They don't have the complex cell structure of the Eukarya. The Eukarya are pretty nifty - they weren't discovered until the 1970s - and they can survive in temperatures up to 90 C.
A pathogen is an agent that causes disease. In particular, certain types of bacteria.