Cavities form when acids produced by bacteria in your mouth dissolve the minerals in your tooth enamel faster than your body can replace them. This process can take months or even years, and it starts long before you’d notice any pain or visible damage. About 21% of American adults have at least one untreated cavity right now, making it one of the most common chronic conditions in the country.
What Happens on Your Teeth Every Day
Within seconds of brushing your teeth, a thin film of proteins from your saliva coats every tooth surface. This coating, called a pellicle, is harmless on its own, but it gives bacteria a surface to latch onto. Within about 24 hours, bacteria colonize this film and begin building a structured community known as a biofilm, or plaque. If you don’t remove plaque by brushing twice a day and flossing, it thickens and matures into a more organized layer that’s harder to dislodge.
The bacteria living in plaque feed on the same sugars and starches you eat. When you consume something sweet or starchy, bacteria (especially one species called Streptococcus mutans) break down the sugars and produce lactic acid as a byproduct. This acid soaks directly into the tooth surface underneath the plaque. What makes S. mutans particularly damaging is its ability to store sugar inside its own cells as a reserve. Once the sugar in your mouth is gone, these bacteria continue breaking down their internal stores and releasing acid, extending the attack well beyond your last bite of food.
S. mutans also uses sugar to build a sticky, glue-like scaffold around itself. This scaffold helps the bacterial community cling tightly to teeth and shields it from saliva, which would otherwise wash the bacteria away.
The Acid Attack on Enamel
Your tooth enamel is made primarily of a mineral called hydroxyapatite, a crystalline structure built from calcium and phosphate. When bacterial acid lowers the pH at the tooth surface below a critical threshold, calcium and phosphate ions start dissolving out of the enamel crystals and into the surrounding fluid. This is demineralization.
That critical pH sits around 5.5 for most people, though it varies. If your saliva is naturally low in calcium and phosphate, your enamel can start dissolving at a pH as high as 6.5. If your saliva is mineral-rich, your teeth may resist acid down to about pH 5.1. This is why some people seem more cavity-prone than others despite similar diets and brushing habits.
Each time you eat or drink something containing sugar, the pH in plaque drops within minutes and can stay low for 20 to 30 minutes or longer. If you snack frequently throughout the day, your teeth experience repeated acid attacks with little recovery time in between.
Your Mouth’s Built-In Repair System
Saliva is your primary natural defense against cavities. It works in three ways: it physically rinses food particles and bacteria off teeth, it neutralizes acid, and it supplies the raw materials your teeth need to repair themselves.
The neutralizing power comes mainly from bicarbonate, a buffering compound that rises dramatically when you chew or taste food. Resting saliva contains about 1 millimole per liter of bicarbonate, but stimulated saliva can contain over 50 times that amount. Phosphate ions and the breakdown of proteins into alkaline compounds like ammonia also help bring the pH back to safe levels.
Once the acid is neutralized, saliva delivers calcium and phosphate ions back to the tooth surface, where they can redeposit into the damaged enamel crystals. Saliva keeps these minerals at concentrations higher than what’s needed to maintain enamel, creating conditions that favor repair over destruction. This back-and-forth between mineral loss and mineral gain happens constantly throughout the day. Cavities only develop when the balance tips toward loss over an extended period.
From White Spot to Full Cavity
The earliest visible sign of a cavity is a white spot lesion: a small, chalky patch on the tooth where minerals have been lost from beneath the surface but the enamel hasn’t physically broken down yet. At this stage, the damage is completely reversible. If conditions in the mouth shift back toward remineralization (through better brushing, less frequent sugar exposure, or fluoride use), the minerals can re-enter the enamel and the lesion can heal.
If the acid attacks continue, the weakened enamel eventually collapses inward, creating a physical break in the surface. This is when a white spot becomes an actual cavity. Once there’s a hole, bacteria can move deeper into the tooth where a toothbrush can’t reach, and the process accelerates. The cavity may still be limited to the enamel at first, but decay typically progresses into the softer layer underneath called dentin. Dentin is less mineralized than enamel, so it breaks down more quickly once exposed.
Left untreated, bacteria continue working inward until they reach the pulp, the soft tissue at the center of the tooth that contains nerves and blood vessels. Infection of the pulp causes significant pain and can lead to an abscess, a pocket of infection at the root of the tooth. At this point, the tooth usually needs either a root canal or extraction.
Why Certain Teeth Are More Vulnerable
Not all tooth surfaces carry equal risk. The chewing surfaces of your back teeth (molars and premolars) have natural grooves, pits, and fissures that trap food and bacteria. These grooves make up only about 12.5% of all tooth surfaces, yet they account for 88% of cavities in children. Some people have deeper, narrower grooves that are nearly impossible to clean with a toothbrush, making these teeth especially prone to decay.
Areas between teeth are also high-risk zones because plaque builds up in tight spaces that brushing alone can’t reach, which is why flossing matters. The gum line is another vulnerable spot, particularly as gums recede with age and expose the tooth root, which lacks the thick enamel layer that protects the crown.
How Fluoride Changes the Equation
Fluoride shifts the balance toward remineralization in two important ways. When fluoride is present at the tooth surface (from toothpaste, treated water, or professional treatments), it gets incorporated into the repaired enamel crystals, creating a modified mineral that is more acid-resistant than the original. This means teeth that have been exposed to fluoride can withstand a lower pH before they start dissolving.
At the higher concentrations found in toothpaste, fluoride also triggers a second reaction. A small amount of enamel dissolves, releasing calcium ions that immediately combine with the fluoride to form calcium fluoride deposits on the tooth surface. These deposits act as a slow-release mineral reservoir, gradually releasing fluoride and calcium over time to keep the repair process going between brushings.
This is why fluoride toothpaste is effective even though you spit it out. The small amount left behind on your teeth continues working for hours. For the same reason, dentists often recommend not rinsing your mouth immediately after brushing.
What Tips the Balance Toward Decay
Cavities aren’t caused by any single factor. They develop when several conditions overlap: frequent sugar or starch exposure gives bacteria fuel, inadequate brushing lets plaque build up, and insufficient saliva flow reduces your mouth’s ability to neutralize acid and deliver repair minerals. Dry mouth, whether from medications, medical conditions, or simply not drinking enough water, is one of the strongest risk factors for rapid decay.
The frequency of sugar exposure matters more than the total amount. Sipping a sugary drink over two hours causes far more damage than drinking the same amount in five minutes, because it extends the window of acid production. Sticky foods that cling to teeth, like dried fruit or caramel, also prolong the acid attack compared to foods that wash away quickly.
Children and older adults face the highest risk at different ends of the spectrum. Nearly 18% of children aged 6 to 8 have untreated decay in their baby teeth, while about 13% of adults over 65 have untreated decay in permanent teeth, often related to gum recession and dry mouth from medications.