Sicknesses (8)
I believe the answer is:
diseases
'sicknesses' is the definition.
(disease is a kind of sickness)
This is the entire clue.
(Other definitions for diseases that I've seen before include "Illness brought on by infections or diet" , "Afflictions, disorders" , "Disorders, illnesses" , "Illnesses, maladies" , "Sicknesses" .)