Abnormal susceptibility is very good friend of disease. Without it, disease cannot exists. They cannot exist without the presence of another, such a good friendship both of abnormal susceptibility and disease share. That is, if there is a disease occurring inside human body of an individual, then you will definitely find abnormal susceptibility occurring inside human body of that individual. Image Source - https://commons.m.wikimedia.org/wiki/File:Differentialsusceptibilitymodel.svg