Formula milk

Hi I’m using cuckoo water dispenser. I fully BF for 3months & now with Nan ha. For mixing milk I need to use room temperature and hot water (40 degree) My baby easily catch flu even after medication. Why? When getting room temperature I realised that the dispenser casually mixing the hot and cold water together. Is this will cause flu to my baby? Her flu is non stop.. I’m wondering why.. please help me. I checked and coway is not like that as it provide room temperature w/o mixing.