Common causes include damage to the hair cells in the ear, damage to the bones in the ear, or a damaged eardrum. Exceesive ear wax or nerve damage can also lead to hearing loss.
I have been treating a parasite in my head. There are times when my ears clear up, then it goes away. This makes me think that parasites could do this. My grandmother and father probably both have parasites and both can't hear worth a darn.