Con Crud is any disease you get after a convention, typically colds, flu and other contagious respiratory viruses, but sometimes intestinal disorders or food poisoning.
A weekend spent in close proximity to a lot of people and their germs, usually without enough sleep, is the common culprit, although such typical fanac as sharing bottles, open bowls of snacks and other substances; snogging; and getting stressed out also contribute.
In recent years, some concoms have tried to lessen the spread of con crud by supplying consuite snacks in single-serving packages or dispensers, but based on anecdotal evidence, it hasn't helped much. The usual precautions — wash your hands frequentl and eat and sleep enough and avoid cons when your immune system is low — are pretty much all you can do to keep germs and viruses at bay. For the sake of your fellow fen, don't go cons when you are sick and cover your mouth when you sneeze!
A convention has been defined as "an opportunity to get together with viruses from all over the world."