Cavity
Decayed Tooth, Tooth Decay
noun, の adjective
A tooth bug... Luckily, it's not some bug living in your tooth, though you can see why someone might think this is what happens. A tooth with a "bug" in it is a tooth with a cavity.
The readings are both kun'yomi, but those are the readings you learned with these kanji. Notice that は turns to ば, though. Presumably this is because you might yell out "BAH!" when you find out you have a cavity.
あっ、虫歯が三本もありますよ。
Oh, you have three bad teeth.
また虫歯になっちゃったの？
Do you have a cavity again?
虫歯は予防が大切です。
Prevention is the key to tooth decay.