Can an untreated cavity in one tooth spread to another?

Yes: It is most common when the cavity occurs on proximal surfaces of teeth. In other words, if the cavity on tooth a is in contact with a surface of tooth b then tooth b will likely form a cavity if tooth a is not treated. Cavities are a bacterial process so good diet and hygiene is essential to reducing risk. A diet high in fermentable carbs and acidity increases your risk in the long run.

Yes: The cavity contains bacteria. If the bacteria come into contact with another tooth for a prolonged period of time, they will start a cavity on that tooth as well. It happens very often in the area in between the teeth, when lack of flossing leaves bacteria in the surface of a tooth that starts a cavity, and as that cavity goes untreated, it can extend to the adjacent tooth.

Absolutely: For this to happen requires some time, however. Important that you 1) floss daily, and 2) see Dentist at least twice a year to treat cavities when they are small, easy to treat, less expensive. Talk to Dentist about how to alter diet, oral hygiene, to prevent recurrence.