Here's a great clip of Steve Rinella talking to the author of Charchutrie about parasites in bear meat:
http://www.youtube.com/watch?v=h2ulsLOPgNQ
Trichinella spiralis is member of the roundworm family. You get infected by eating meat from another infected animal's meat, so only carnivores or omnivores can get it. The worm is released from the meat by the digestive enzymes in your stomach and the adult worms then burrow into the epithelial lining of your small intestine where they mate and produce new larvae. The larvae then burrow through your tissue into your blood and lymph vessels where they spread throughout your body and deposit into your skeletal muscle where they encased themselves and persist for years. Sometimes the larvae get lost and deposit into your heart muscle or brain which will kill you. When another animal eats that meat, the life cycle is completed. Yeah, crazy gross!
The reason it used to be so prevalent in pork meat was because they used to feed pigs pork scraps, thus reinfecting other pigs. The banning of feeding pigs uncooked meat scraps has essentially eliminated this phenomena in pork, but that is why they always tell you to cook pork well done all the time.
The state will usually test a sample for you. When I shot a bear in Montana, they offered to take a sample from the tongue to test. But even though the state will exempt you from wanton waste laws if it is positive, you can still eat its meat as long as you cook it well done. A combination of freezing your bear meat and cooking it well done every time would be a pretty good combo to make sure you won't get sick. I subscribe pretty heavily to the idea that you should do all you can not waste anything you have killed and would honestly still eat an infected bear.
