You slurped the oysters, devoured the pork chop and ate every crumb of the apple pie -- yet another fine dinner at a favorite restaurant. But at 3 a.m., you awake in a sweat, your insides churning and one thought on your mind: "That [expletive] restaurant made me sick."

Not so fast. You're sick all right. But was it the oysters? Or that succulent pork chop? Can you even be sure the culprit was part of your meal?

"It's like someone telling you they got a cold from riding the 22 Fillmore," says Carlo Middione, chef-owner of Vivande in San Francisco. Most diners in gastric distress instinctively blame the last place they ate, but it's not necessarily that simple.