Your investigation skills pay off! After talking with the kids and parents, you discover that the patients did not eat the same foods and quickly rule out foodborne illness.
Test results for 9 sick kids who attended the party came back positive for Cryptosporidium (or "Crypto"). The health department puts out an alert to encourage testing for Crypto (since it's not routinely done in most laboratories). Soon after, a local hospital reports 13 new Crypto infections in patients who did not attend the birthday party.