Baby bottle tooth decay is caused by prolonged contact with almost any liquid other than water.  This can happen due to putting a baby to bed with a bottle of formula, milk, juice, soft drinks, sugar water, sugared drinks, etc.  Allowing the baby to suck on a bottle or breastfeed for longer than a mealtime, either when awake or asleep, can also cause baby bottle tooth decay.
Some ways to avoid this problem include: never putting your child to bed with a bottle, giving your baby a bottle during meals only, keeping your baby’s mouth clean and teaching you child to drink from a cup.