The Pennsic Plague is a term in the SCA for various gastrointestinal difficulties, usually encountered a few days into a long event (such as the two-week long Pennsic War.) Symptoms start with loss of appetite and general listlessness and progress quite quickly to diarrhoea, stomach ache, nausea, vomiting.
Many theories have been forwarded for the causes of Pennsic Plague, including change of diet, stress, heat stroke, sudden exposure to an entire continent's worth of potentially diseased strangers, inadequately filtered water, disease-bearing insects, excessive alcohol consumption and/or failure to wash ones hands properly after using the privies.
While the Pennsic Plague is generally not life-threatening it can seriously effect one's enjoyment of an event. Chirurgeons generally recommend taking particular care with sanitary habits, such as throughly washing with soap and water, keeping utensils clean and making sure that food is covered to keep off flies. If one is stricken by the Plague, victims are encouraged to drink plenty of fluids and get bed rest -- the effect pass in two or three days.