Many clinicians diagnose disorders by using criteria listed in the DSM-5. The DSM-5 is the abbreviations for the diagnostic and statistical manual of mental disorders, it is the handbook used by the healthcare professionals as the guide to the diagnosis of mental disorders. It contains descriptions, symptoms, and other criteria for diagnosing mental disorders.
Answer:
Chemical properties are harder to change than physical properties.
The stronger survived and the weaker passed on... Those with a weaker immune system had trouble with sickness and usually ended up passing on but those with strong immune systems were stronger and healthier and lived.
I believe it is idiopathic.