The oral cavity consist of the lips, cheeks, tongue, teeth, gum,  various structures attached to it and salivary gland which is responsible for the secretion of watery fluid called the saliva. Different disease and malformations can occur in any of these structures of the oral cavity, even though certain disease are more common among younger age  while others are common at older age.
There is various classification of oral disease, some of which include:
There are millions of bacteria that live in the mouth which posses a great threat to the body immune system which when lowered due to Illness or other factors may affect the alveolar bone, teeth, gum and other factors.
The lips, cheeks, teeth, tongue, and bony structures suffer from different trauma’s .some of which result from fall, fight and road rage causing fracture, tear, and crush of tissues concerned in addition to bleeding that may accompany the injury.
Tumor or abnormal growth can affect the mouth and its contents. Common risk factors are cigarette smoking or tobacco chewing, excessive alcohol consumption.

These are disease that arises as a result of old age such as natural tooth loss. Also, decreased salivary flow could predispose to dry mouth which results in dental caries commonly known as tooth ache.
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This is commonly referred to as dental caries. It is one of the most common oral health problems. It is a slow process involving the localized destruction of the teeth often resulting into a hole or cavitation in the affected tooth/teeth. The process involves continuous demineralization of the tooth due to by-products of microorganism on fermentable carbohydrate in the individual’s tooth. Some bacteria in the dental plaque [an invisible mass of soft or yellow harmful layers of germs that live in the mouth and stick to the surface of the teeth. note: plaques are bacteria and not food particles.] are able to turn the sugar we eat into acid, which when slowly dissolves the tooth –first from enamel to dentin leading to formation of a hole and built of pains after a cold food or drink.
Tooth decay affects about 90-95 percent of total world population i.e. it is a pandemic. Similarly, the prevalence was high in whites but presently the reverse is the case.

Pains at advanced stage include dark spots on the teeth which sometimes comes with fever [if infection spreads to other parts of the body – i.e. becomes systemic], swelling [if there is collection of pus], hole or complete loss of teeth, bad odor from the mouth.

Primary prevention: this involves the prevention of the said disease in an individual before it happens at all through general health promotion, health education, health maintaining habits, good nutrition, oral hygiene ,good eating habits, fluoridation, regular dental check-up and stopping/quitting certain habits.
Secondary prevention: this is aimed at halting the progress of the disease at its early stage before serious or irreversible damage occurs. This is done through early diagnosis and prompt treatment.
Tertiary prevention: sometimes bad damage would have occurred and what could be done is limitation of the disability e.g. counseling to quit tobacco, smoking or desist from sugary consumption and also rehabilitation.