Tooth decay or a decayed tooth (dental cavity); the condition caused by bacteria damaging the tooth.
虫歯で歯医者に行かなければならない。
I have to go to the dentist because of a cavity.
定期検診で虫歯が見つかった。
A cavity was found during the regular checkup.
甘い物の食べ過ぎは虫歯の原因になる。
Eating too many sweets can cause cavities.