This is a softening of tooth structure caused by the acid which is produced by oral bacteria from the sugar that we eat or drink.
Some people are more prone to decay than others, but we can all reduce our risk as follows:
Consume sugar less often. Frequency is all-important. As an example, one cup of coffee per day with ten spoons of sugar in it will cause less decay than ten cups per day, each with one spoon of sugar.
Brush in the morning and at night with a fluoride toothpaste.
After brushing, do not rinse with water. Leave the traces of toothpaste on the teeth.
After brushing at night, do not consume anything except water until morning. Sugar on the teeth during the night causes high decay risk because saliva flow dries up when we sleep. There is therefore no dilution of acid on the teeth during the night.
Chew sugar-free gum for ten minutes after consuming sugar. The resulting prolonged saliva flow may lower decay risk.