Hi Susie, simple explanation is your body releases hormones to fight infection in your body and side effect is high blood glucose readings. If I remember correctly if your running above 18 mmol/l you should check for ketones, which is why us type 1's have to be extra vigilant with illness to ensure we don't get ketoacidosis, lots of testing and corrections and follow sick day rules if you've done DAFNE, otherwise check with you diabetic team.