The National Association of the Deaf (NAD) defines deafness as the "audiological condition of not hearing." The NAD includes people with very limited hearing who cannot rely upon it for comfortable communication.
Deafness can be caused by:
- heredity - genetic links have been found in the Human Genome Mapping Project;
- severe illness in childhood such as rubella (German measles) or spinal meningitis;
- prenatal maternal illnesses;
- exposure to long-term loud noise or exposure to sudden extreme noise;
- preventable prenatal substance abuse;
- physical injury or damage to the brain, the head, or the ear;
- age-related hearing loss.