diabetic:
(plural: ‘diabetics‘; pronunciation: the letter ‘c’ is said with a ‘k’ sound as in “king”)
a person suffering from diabetes; a sugar patient,

a young diabetic taking an insulin injection
In some places a diabetic is called a sugar patient because diabetes is a disease that is related to sugar levels in a person’s body.
My grandmother was a diabetic, and so, my physician has been worried I may inherit diabetes from her.
There are some unfortunate diabetics who suffer from diabetes though they none of their family members has or had diabetes previously.
In the past, diabetics had a tough time going to a clinic to get their blood sugar levels checked or to get a dose of insulin, but these days there are hand-held apparatus/device with which a patient can check his/her sugar levels in his/her blood anywhere very easily, and then, depending on the reading in the device, he/she can take oral dose of quick acting pills!
