Yes to all your questions. They parasitize brood, weakening them and causing loss of vigor. They also vector viruses, including Deformed Wing Virus (shriveled wings) which is a late sign of impending disaster. I don't think any one mechanism is the big bad wolf, but rather the sum of the stresses that a large infestation inflicts on a colony. Which can cause mortality outright, but is more commonly (I suspect) a contributor to a colony being vulnerable to other diseases, robbing, overwintering poorly, etc.
Bees, brews and fun
in Lyons, CO