Something not noted yet in this quick primer... the issue of plague.

Normally when plague happens, one or more of your characters is a carrier. It is paramount that you isolate that character from same stack contact with any other units. If you right click on the character's card, it will show a skull in the upper left of the popup that says 'plague carrier'. Isolate that boy. Normally you will lose troops for the first couple of turns, then the major portion of the plague will be over.