The body responds to illness in the same way as it does to stress and this results in the blood glucose increasing during illness. It is possible that the blood glucose rises even before symptoms are noticed.
Unfortunately, if blood levels rise any infection can take hold quicker so it is important to understand sick day rules when you have diabetes. There are also important rules with certain medications during illness particularly if the illness is associated with sickness or diarrhoea.
For the details on how to look after yourself if during illness please follow this link:-