Deafness is the complete inability to hear sound. It can occur at any age. A person may go deaf all of a sudden due to an accident or a severe viral infection, or gradually over time as a result of disease, nerve damage or injury caused by noise. Ear infection, fluid build-up behind the eardrum, hole in the eardrum, genetic disease and birth defects can also result in hearing loss. Deafness is associated with age as well. About 3 in 1000 babies are born deaf.

Color-blindness is color vision deficiency that is an inability or reduced ability to differentiate between colors. It is a condition in which a person is unable to perceive color differences under normal light. Ishihara test is done using different number plates to find if the person is color-blind. This condition arises due to a fault in the development of retinal cones. Some types of color-blindness are red-green, which is the inability to distinguish between red and green colors; blue-yellow, which are the inability to distinguish blue and yellow colors, and total color-blindness.