What is 'disease'?

I still remember the first internal medicine lecture delivered by Dmitry Nikolaevich Emelyanov.

Lecturer: It is not easy to define the word 'disease'. What is 'disease'?
Student : Deviation from normal.

Lecturer: Yes correct, so what is normal? *Taking down his glasses* 

                  If I need to wear glasses, am I considered as abnormal or having disease? 

                  *Putting back his glasses* So it is rather hard to define what is 'disease'.