What happens if cavities are left untreated?
On the "dangers" of tooth decay and dental infections not treated by a dentist
"What can happen if I don't get a cavity filled?" "Is it dangerous when a carious tooth is not treated in a dentist's chair, as dentists affirm?" "Does the jaw get infected and can a life-threatening abscess form?" Since the following attempt at answering these questions touches upon controversial issues, the reader's attention is expressly drawn to this site's Disclaimer.
With respect to untreated tooth decay in a tooth that has never received dental treatment before, Dr. Lars Hendrickson (a trained medical doctor and dentist) addresses some of the above questions in his book Zahnarztlügen ("Dentists's Lies", which advises total dental autonomy).
To begin with, there are no studies available showing the effects of tooth decay remaining untreated (by a dentist). Consider however the following:
In Germany alone, there are sixteen million people who (even with extreme toothache) never go to the dentist. Since 95 percent of the currently roughly 80 million Germans do have dental caries, totalling a number of 76 million affected, there are a minimum of twelve million Germans with tooth decay who refuse to see the dentist. If if were true that life-threatening abscesses are likely to form, some of these twelve million people should regularly be reported to have died upon developing a death-dealing abscess from an untreated dental infection or caries. Since this is not the case, it can clearly be deduced that fatal abscesses are extremely rare (as indirectly confirmed by the twelve million+ surviving Germans who never see a dentist in spite of suffering from tooth decay).
How tooth decay progresses when unchecked and untreated
What typically happens when a tooth gets infected and no proactive countermeasures such as dietary upgrades are taken (particularly cutting out all sugar and junk food), using xylitol and chlorhexidine [and many other things, see particularly Healing Teeth Naturally's Natural, holistic & home dental remedies and Dental Care & Oral Hygiene sections]), is really quite unspectacular.
The caries (tooth decay) will gradually progress, with the tooth slowly becoming sensitive to hot and cold. Months into this process, the first pain will make its appearance. Then a short phase of (in most cases) strong pain will occur, followed by the death of the nerve and the cheek starting to swell. Nature in its wisdom proceeds to create a pus pocket which it soon after empties, thus voiding dead bacteria and pulp matter. According to Dr. Hendrickson, this uncomplicated series of events is typical, with some not even noticing the demise of their tooth, i.e. not even going through the short period of "hellish pain" predicted by the dentists.
In other words, according to Dr. Hendrickson, in this day and age the occurrence of a life-threatening abscess will be an absolute rarity since well-nourished people with well-functioning immune systems, and anti-inflammatories, painkillers and antibiotics within easy reach, will give bacteria a hard time (rather than the other way round). So according to the same source, warnings that one could die from an untreated tooth are merely scare tactics employed by dentists in sheep's clothing who will gladly conjure up horror scenarios as to what might happen if caries was left untreated by their profession.
Self-treatment to check further progress of tooth decay
To prevent tooth decay from progressing as above described, Dr. Hendrickson advises to cut out all sugar, eat a balanced diet, clean your teeth properly, use xylitol and particularly when there are fillings present, also chlorhexidine and high-dose chlorhexidine gel to protect your teeth from developing secondary caries (for an explanation of this term, see point 5 under Drilling & filling teeth: an unwise choice?).
So much for Dr. Hendrickson's observations and advice (summarized in English and with notes and links added by Healing Teeth Naturally).
When a tooth pre-treated with a filling or root canal develops new tooth decay which is left untreated (by a dentist)
This may be quite a different story since the body here not only has to contend with something it is used to - simple bacteria, their metabolic by-products as well as with dying and dead tissue (here: tooth pulp). In contrast, here the body faces a number of additional aggravating circumstances and aggressors - toxic metals and/or other noxious dental materials as well as anaerobic bacteria of possibly increased virulence. While the body can deal with these too, it is - as next to always - a question of...
The power of the immune system (resistance) versus the power of germs/bacteria (aggressors)
A body and mouth that is optimally supported by nutrition, regular detoxification, immune-supporting herbs and supplements, natural antibiotics, a stress-free lifestyle, oral urine treatment etc. etc. seems much better able to deal with dental toxins of various origins. A body and mouth which does not receive proper support may be less able to contain and successfully overcome virulent dental infections.
What happens after the tooth pulp has died?
(The following scenario leaves aside any potentially reviving influences on the pulp from strict dietary measures as shown in this Gerson diet testimonial as well as from spiritual practices as featured in the Spirituality section of this site.)
When the tooth pulp containing the nerve and blood vessels carrying nourishment has died, the tooth is dead and the pulp chamber will be emptied of all content at some point. Depending on various factors such as the structural strength of the remaining tooth shell, oral acidity and plaque levels and frequency of using the tooth for chewing hard items, the tooth should eventually be worn down to the gum line and finally (likely) vanish altogether from the mouth.
How about a decaying tooth creating a focal infection which might create diseases in other parts of the body?
The highly complex theory (and practice) of focal infections (when a localized infection such as in the jawbone may lead to disease symptoms elsewhere in the body) will be treated under a separate heading.
Conclusion: should I leave my cavity untreated (by a dentist)?
As implied above, dealing with an infected tooth by yourself or in the dentist's chair is your call. Personally I know people who have opted for the former including myself (and only being able to speak of me personally, with no untoward but as far as I can tell only positive results).
Should you opt for self-treatment or the dental autonomy route, a good place to start your journey is Advice on what to do when you can't or don't want to see a dentist and of course the entire Dental self healing, regeneration & regrowth of teeth and gums section.