Knowledge of disease is necessary because if we do not know what is called disease, then how would you identify it? How would you recognize if a disease is occurring inside human body or not? Disease can be identified through its symptoms , its manifestations, but if you do not know what are normal reactions of human body and what are disease manifestations, then how would you identify them and differentiate them? Image Source - https://commons.m.wikimedia.org/wiki/File:Main_symptoms_of_Infectious_mononucleosis.png