Per the Centers for Disease Control & Prevention (CDC), Cyclosporiasis is an intestinal illness that "causes watery diarrhea, with frequent, sometimes explosive, bowel movements. Other common symptoms include loss of appetite, weight loss, stomach cramps/pain, bloating, increased gas, nausea, and fatigue. Vomiting, body aches, headache, fever, and other flu-like symptoms may be noted. Some people who are infected with Cyclospora do not have any symptoms."
Symptoms can sometimes take a week to appear and are treated with antibiotics, according to the CDC.
Although a source of the scourge hasn't been traced back to a single McDonald's location, the Illinois Department of Public Health notes that many victims reported eating a McDonald's salad the day symptoms were reported.