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.
An update is needed before you can continue but you have more than one WaniKani tab open.
Finish up what ever you are doing on the other tab(s), then close them to continue.
The update will complete automatically once the other tab(s) are closed.