Why Rare Steak is Safe - but NOT Rare Chicken 

Rare steak is pretty safe to eat, because the germs of greatest concern are usually found on the surface and will be killed when seared. But, as Dr. Francisco Diez-Gonzalez of the University of Georgia's Center for Food Safety explains, research shows similar germs can live deep inside the muscle of chicken, and therefore can't be killed by simply searing the exterior. Ground beef is more dangerous than whole cuts because the exterior has been mixed into the interior. Fish is more complicated (depending on the species), but very few people get sick from eating totally raw fish (i.e. sushi). High-quality pork can be safe when cooked pink, as the U.S. Department of Agriculture (finally) acknowledged in 2011.
SOURCES
Dr. Diez-Gonzalez: foodscience.ca...
CDC report documenting the decline of trichinosis in the U.S., and the prevalence of cases involving non-pork sources, such as wild game: www.cdc.gov/mm...
USDA's 2011 revised guidance on cooking pork: www.usda.gov/m...
All photos of pathogens are public domain, drawn from Wikipedia.