Wasn't it something to do with the feed. Rare pork could give you worms (that could be all in my head) Chicken you can salmonella. Beef seems to be fine....as long as they don't have mad cow disease.
Pork used to contain tape worm eggs/larvea that could be transmitted to humans. Now they deworm the pigs so that makes it safer, but actually I think its more to do with taste -pork tastes much nicer well-cooked.
I have heard that all chickens contain salmonella in the UK (that's why they dip the chicken in a weak bleach solution in the USA), and again, raw chicken would not taste as nice as something well-cooked.