Today, I had my decayed tooth extracted at the dentist.
Streptococcus mutans bacteria, the cause of tooth decay, can be transferred from adults to the kids around them as they grow up.
I have so many cavities but I don’t have dental insurance and I can’t afford to get them treated.
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.