Hearing is one of the five senses, along with vision, taste, smell, and touch. The ears initially receive sound waves, and, through a series of processes, transmit them to the brain. In the brain, the sounds we hear are interpreted, integrated, and stored. In order to properly function, the auditory system requires a source of sound, a mechanism for transducing this sound into useful information, a mechanism for relaying the information to the brain, and pathways in the brain to process the information.
For most people, these mechanisms work well enough--but occasionally, something goes terribly wrong. According to World Health Organization, 360 million people worldwide have disabling hearing loss; a figure that amounts to 5% of the global population. Hearing loss is the decrease in hearing sensitivity to any degree. Hearing impaired people can be deaf (a condition determined by a profound or total loss of hearing in both ears) or hard of hearing (HOH). 
Deafness can be congenital or acquired. The distinction between acquired and congenital deafness specifies only the time that the deafness appears, but not the cause of the deafness.
In congenital cases, an individual acquires the deficiency either neonatally or postnatally. Hearing loss can be caused by various hereditary and non-hereditary genetic factors, or by certain complications during pregnancy and childbirth, including premature childbirth, hospital infections, use of forceps in childbirth, birth asphyxia (a lack of oxygen at the time of birth), inappropriate use of particular drugs during pregnancy (such as aminoglycosides, cytotoxic drugs, antimalarial drugs and diuretics), or a low birth weight. In addition, diseases affecting a pregnant mother can be transferred to the baby, and potentially cause severe jaundice in the neonatal period, which can damage the hearing nerve in a newborn infant.
Unlike congenital cases, acquired cases may lead to hearing loss at any age. Acquired cases can be caused by infectious diseases such as meningitis, measles, and mumps; chronic ear infections; and an abnormal buildup of fluid in the ear (otitis media). The following can also contribute to the onset of hearing loss: use of particular drugs, such as otoxins as well as, some antibiotics and antimalarials medicines; injury to the head or ear; excessive noise, including occupational noise such as that from machinery and explosions, and recreational noise such as that from personal audio devices, concerts, nightclubs, bars and sporting events; ageing, in particular due to degeneration of sensory cells; and wax or other foreign bodies blocking the ear canal.
Figure Ia. Word cloud from the article text by Lorrayne Isidoro
The classification of a case of hearing loss depends on what part of the auditory process fails to work properly. The main types of hearing loss are categorised as follows:
Sensorineural hearing loss
This type of hearing loss occurs as a result of injury to sensory cells of the cochlea (found in the inner ear) or nerve (auditory nerve) that leads the sound stimulus to the brain. The diseases of the cochlea and the auditory nerve rarely have treatment.
The use of ototoxic drugs (that damage the hearing aid) can also lead to deafness. Some of them are antibiotics, aminoglycosides and salicylates. So the self-medication should not be made under any circumstances.
Conductive hearing loss
When we say that hearing loss is by conduction, that means there is something blocking the passage of sound from the outer ear to the inner ear. This can result from damage to the eardrum, excess wax that accumulates in the ear canal, or the introduction of some material into the ear canal. Infection in the middle ear ossicles can also cause conduction deafness. This type of hearing loss can be reversed by drugs or surgery.
Mixed hearing loss
It occurs when there are problems in both the inner ear or auditory nerve and the middle or outer ears. This results in raised air- and bone-conduction thresholds, as well as an air–bone gap.
Central hearing loss
Another type of hearing loss is called central deafness. It occurs as we age and is part of a natural process of the body. Just like the eyes and the heart, the auditory system of the person also suffers wear and tear over the years. How the person treats the ears has an influence on presbycusis, the name given to aging-related hearing loss.
One of the main impacts of hearing loss is on the individual’s ability to communicate with others. Spoken language development is often delayed in children with deafness.
Hearing loss and ear diseases such as otitis media can have a significantly adverse effect on the academic performance of children. However, when opportunities are provided for people with hearing loss to communicate, they may be able to participate on an approximately equal footing with others. The communication may be through written language or through sign language.
Sign language is a manner to communicate without using spoken language. Sign language employs hands shapes, orientations, and movements; facial expression; and body language. Sign Language is used mainly by people who are deaf or have hearing impairments.  It is worth mentioning that sign languages are not the same in the countries--it is not a universal language. Each country makes their own sign languages, similarly to how countries treat spoken languages. They are developed naturally out of groups of people interacting with each other. For example, in American Sign Language (Figure 1b), the alphabet is signed with one hand. But in British Sign Language (Figure 1c) is with two! 
Figure Ib, 1c: American vs British Sign Language
Half of all cases of hearing loss can be prevented through primary prevention. Some simple strategies for prevention include: immunizing children against childhood diseases, to prevent diseases that have contact with leave sequelae such as deafness; Do not take ototoxic drugs without a prescription; improving antenatal and perinatal care, including promotion of safe childbirth; reducing exposure (both occupational and recreational) to loud sounds by raising awareness about the risks; and encouraging individuals to use personal protective devices such as earplugs and noise-cancelling earphones and headphones.
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