Let me try to explai. A cavity is decaying tooth material. The tooth can abscess and infection can spread to the tissues surrounding the tooth. Some of the tissues surrounding the tooth are the gums. Abscesses form, reddness and inflammation occur and a serious condition then exists. See your dentist.
No . Cavities are consequence of eating too much sweet , poor oral hygiene, not regular brushing . Cavities don't migrate but other teeth can also develop cavities due to same predisposing factors. .
Body is Connected. The whole body is connected cavities lead to other health issues! among them gum & periodontal issues. Stop cavities at the 1 surface of decay level if possible. Do not allow them to spread. Prevention is inexpensive, compared to repairing large dental, gum & health issues. Also, less painful & less time consuming. See a dentist & develop a good relationship with a visit every 6 months.