Ill the body and the level of sugar in the blood.. Colds and fever stimulates the body to secrete certain hormones that raise blood sugar level

Physical illness such as colds and such things stimulates the body to secrete certain hormones that raise blood sugar level. 

In addition to that, the fever, for example, increases the metabolism of the body and increase the use of the body with sugar and therefore change the amount of insulin needed by the body in the natural state. 

So you must control the sugar level regularly during periods of fatigue.