Why do we get cavities in our teeth?

A sticky film of bacteria called plaque constantly builds up on teeth. When we eat sugars and starches, bacteria such as Streptococcus mutans ferment them and release acids. These acids lower the pH at the tooth surface and pull minerals out of the enamel, a process called demineralisation. Saliva and fluoride help rebuild enamel, but if acid attacks happen too often the damage outpaces repair and a hole forms. Tooth decay is one of the most common diseases in the world, with untreated cavities in permanent teeth affecting around 2 billion people.