McDonald’s pulled salads from 3,000 restaurants in the Midwest after health experts announced that more than 100 people had been infected by an intestinal parasite in recent weeks.

Public health officials in Illinois and Iowa have reported a surge in cases of cyclosporiasis, with at least 15 infections in Iowa and 90 others in Illinois. Everyone who became ill in Iowa and about a quarter of those who became sick in Illinois said they had eaten McDonald’s salads in the days before symptoms appeared, according to the states’ health departments.

“Although a link has been made to salads sold in McDonald’s restaurants in some Illinois cases, public health officials continue to investigate other sources,” Dr. Nirav D. Shah, the director of the Illinois Department of Public Health, said on Thursday.