How decay develops
Bacteria in the mouth feed on food residue, especially sugar, and produce acid. The acid draws calcium out of the enamel. At first this is just a white spot, with no cavity yet. At this stage decay is still reversible — with fluoride and proper hygiene the enamel recovers.
If the process continues, the enamel collapses and a cavity forms. From here there is no way back: damaged tissue does not grow again. The decay reaches the dentine — a layer softer than enamel — and spreads there much faster, often widening inside while the opening on top stays small.
In the final stage decay reaches the nerve: pulpitis begins, with severe pain, especially at night. By then a filling is no longer enough — root canal treatment is needed, which costs 3-5 times more.