A parasite called cyclospora is spreading through Michigan, and health officials still haven't found the source. In a normal year, the state sees about 50 cases; in the last two weeks, it has counted 700 infections among people aged 8 to 84, according to Dr. Natasha Bagdasarian, chief medical officer for the state of Michigan.

It usually rides in on fresh produce — berries, herbs, leafy greens — but so far, no food has been named. Oakland County health officer Kate Guzman, a nurse by training, joins host Robyn Vincent to explain how investigators hunt for an invisible source and what it means that, a year ago, the Centers for Disease Control and Prevention stopped requiring states to report this exact parasite to one of its main early warning systems.

Editor's note: This conversation was recorded before the latest case counts were released. The Michigan Department of Health and Human Services now reports 700 cases of cyclosporiasis. The outbreak is ongoing and the number continues to rise.