A dental carie, more commonly called a cavity, is permanent damage to the hard surface of a tooth caused by bacteria, acids, and sugars working together over time. It’s one of the most common health conditions on the planet, affecting roughly 27.5% of the global population in their permanent teeth alone. The process starts silently, often without pain, and progresses through distinct stages that determine how it’s treated.
How a Cavity Actually Forms
Your mouth is home to hundreds of bacterial species, but one in particular drives most tooth decay. This bacterium thrives on sugars from food and drinks, converting them into acids. It has three traits that make it especially damaging: it produces sticky polymers that help it cling to teeth, it can ferment a wide range of carbohydrates (not just table sugar), and it continues producing acid even in extremely acidic environments where other bacteria would stop.
Those sticky polymers form what you know as plaque, a thin biofilm that coats the tooth surface. Plaque traps acid against the enamel and creates a low-pH pocket that’s difficult for saliva to wash away. Over time, this acid pulls minerals out of the enamel, weakening it. Your saliva naturally works to counteract this by delivering calcium, phosphate, and fluoride back to the tooth surface in a process called remineralization. A cavity forms when the rate of mineral loss consistently outpaces your saliva’s ability to repair the damage.
The Four Stages of Decay
Tooth decay doesn’t happen all at once. It moves through a predictable progression, and the earlier it’s caught, the easier it is to reverse or treat.
Stage 1: White Spots
The earliest sign is a chalky white spot on the tooth surface. This means minerals are leaching out of the enamel but no actual hole has formed yet. At this point, the damage is still reversible. Most people won’t feel anything, which is why regular dental checkups matter. A dentist can spot these white spots before they progress.
Stage 2: Enamel Breakdown
If mineral loss continues, the enamel weakens enough to form a small hole. White spots may darken to brown. This is the point most people think of as a “cavity.” Enamel has no nerve endings, so this stage is often painless too.
Stage 3: Deeper Decay Into Dentin
Beneath the enamel sits dentin, a softer tissue that contains tiny tubes connecting to the tooth’s nerve. Once decay reaches dentin, it speeds up because the tissue breaks down more easily. This is typically when you start noticing sensitivity to hot, cold, or sweet foods and drinks.
Stage 4: Pulp Involvement
At the center of each tooth is the pulp, which contains nerves and blood vessels. When bacteria reach the pulp, it becomes inflamed and swells. Because the pulp is enclosed in a rigid structure, that swelling puts pressure on the nerves and causes significant pain. Infection can develop and spread to the surrounding bone if left untreated.
Why Sugar Matters More Than You Think
Sugar is the primary fuel for acid-producing bacteria, but the relationship is more nuanced than “sugar equals cavities.” What matters most is how often your teeth are exposed to sugar, not just how much you eat in total. Every time you sip a sugary drink or snack on something sweet, you trigger a new round of acid production that lasts about 20 to 30 minutes. Frequent snacking keeps the acid cycle going continuously, giving your saliva no chance to repair the enamel between attacks.
The World Health Organization recommends keeping free sugars below 10% of your total daily calorie intake, and ideally below 5%, to minimize cavity risk throughout life. “Free sugars” includes any sugar added to food and drinks, plus the sugars naturally present in honey, syrups, and fruit juices. Sugars in whole fruits and plain milk don’t carry the same risk because they’re bound up in fiber or protein that slows their release.
Treatment Options by Stage
What your dentist recommends depends entirely on how far the decay has progressed.
In the earliest white-spot stage, a professional fluoride treatment can actually reverse the damage. These treatments deliver a much higher concentration of fluoride than what’s in your toothpaste or tap water, and they come as a varnish, gel, or foam applied directly to the tooth. No drilling involved.
Once a physical hole has formed in the enamel or dentin, a filling is the standard treatment. Your dentist removes the decayed material and fills the space with composite resin (tooth-colored), porcelain, or amalgam. This is the most common cavity treatment, and it’s straightforward for most people.
When decay is extensive enough to weaken the tooth’s structure, a crown may be necessary. This is a custom-fitted cap that covers the entire visible portion of the tooth, protecting it from fracturing. The dentist removes the decayed areas and reshapes the remaining tooth to support the crown.
If decay has reached the pulp, a root canal becomes necessary. The infected pulp is removed, the interior of the tooth is cleaned and sometimes treated with medication, and the space is filled. The tooth is preserved but no longer has living tissue inside it. In the most severe cases, where the tooth is too damaged to save, extraction is the only option. Replacing the missing tooth with a bridge or implant prevents the remaining teeth from shifting out of alignment.
What Actually Prevents Cavities
Fluoride is the single most effective tool for cavity prevention. It works by integrating into the enamel’s mineral structure, making it more resistant to acid attack, and by encouraging remineralization when early damage occurs. Community water fluoridation in the U.S. is set at 0.7 milligrams per liter, a level chosen to maximize dental benefits while minimizing the risk of dental fluorosis (faint white marks on teeth from excess fluoride during childhood).
Brushing twice daily with fluoride toothpaste removes plaque before it can do significant damage. Flossing clears bacteria from the tight spaces between teeth where a toothbrush can’t reach, and where cavities frequently develop unnoticed. Limiting sugary snacks and drinks between meals reduces the number of acid attacks your teeth face each day.
Saliva plays an underappreciated role in protection. It physically rinses food particles and bacteria off the teeth, neutralizes acids, and delivers the calcium and phosphate needed to rebuild weakened enamel. Anything that reduces saliva flow (certain medications, dehydration, mouth breathing during sleep) increases cavity risk. Chewing sugar-free gum after meals can stimulate saliva production and help your mouth recover from an acid attack faster.
Who Gets Cavities Most Often
Cavities are not equally distributed. Children and older adults face the highest risk. Children often have less thorough brushing habits and eat more sugary foods, while older adults commonly take medications that reduce saliva flow and may have receding gums that expose the softer root surfaces of teeth to decay.
Dry mouth from any cause is one of the strongest risk factors. People with naturally deep grooves in their molars are also more vulnerable because plaque collects in those crevices and is hard to clean out. Geographic and economic factors play a role too: populations without access to fluoridated water or routine dental care have significantly higher rates of untreated decay.