Cause of Tooth Decay

Tooth decay is caused over time by plaque forming on your teeth. There are several risk factors which increase the chances of this happening.
A tooth is made up of three parts:

     enamel – the hard outer coating of a tooth
     dentine – the softer, bone-like material underneath the enamel
     pulp – the soft centre of the tooth that contains nerves and blood vessels

How dental decay develops
Your mouth is full of bacteria, which combine with small food particles and saliva to form a sticky film known as plaque, which builds up on your teeth.
When you consume food and drink high in carbohydrates (sugary or starchy), the bacteria in plaque turn the carbohydrates into energy they need, producing acid at the same time.

Over time, the acid in plaque begins to break down the surface of your tooth.
The plaque will first start to erode the enamel. Over time, a small hole known as a cavity can develop on the surface. This will cause toothache.
Once cavities have formed in the enamel, the plaque and bacteria can reach the dentine. As the dentine is softer than the enamel, the process of tooth decay speeds up.

Without treatment, plaque and bacteria will enter the pulp. At this stage, your nerves will be exposed to bacteria, making your tooth very painful. The bacteria can also infect tissue within the pulp, causing a dental abscess.
Tooth decay typically occurs in teeth at the back of your mouth, known as molars and premolars. These are large flat teeth used to chew food. Due to their size and shape, it is easy for particles of food to get stuck on and in between these teeth. They are also harder to clean properly.
It is more common for a front tooth to be affected by tooth decay when it is touching another tooth alongside it.
Increased risk of tooth decay

Factors that increase your risk of tooth decay include:
Diet
     Eating food and drink high in carbohydrates, particularly between meals, will increase your risk of tooth decay.
Tooth decay is often associated with sweet and sticky food and drink, such as chocolate, sweets, sugar and fizzy drinks. Starchy food, such as crisps, white bread, pretzels and biscuits also contain high levels of carbohydrates.

Poor oral hygiene
     If you do not regularly brush your teeth, you are at a higher risk of tooth decay. You should brush your teeth at least twice a day.
Read more about how to keep your teeth clean.

Smoking
     Smokers have a higher chance of developing tooth decay as tobacco smoke interferes with production of saliva, which helps keep the surface of your teeth clean. Studies have also shown passive smoking can be a risk factor, particularly for children.
Read more about how to quit smoking.

Dry mouth
     People who have lower levels of saliva in their mouth are at higher risk of developing tooth decay, because saliva helps to keep the surface of your teeth clean.

A number of medicines and medical treatments can lower the amount of saliva in your mouth. For example:
     • tricyclic antidepressants
     •antihistamines (used in the treatment of allergies)
     •some antiepileptic medicines
     •some antipsychotic medicines
     •beta-blockers (used to treat a number of heart conditions)
     •radiotherapy

If you are taking a medicine or receiving treatment known to cause dry mouth, take frequent sips of still, unflavoured water and make sure you have a good oral hygiene routine.

Thanks for the information: nhs.uk