Plaque is a soft sticky deposit that is excreted by certain types of bacteria which reside in people's mouths. When the bacteria within this plaque digest food such as sugars (simple carbohydrates), they excrete acids and other irritants. Those excretions are what cause tooth decay and gum disease. The adhesive nature of plaque holds the bacteria against the tooth and root surfaces, which is why the bacteria can cause so much damage to your teeth and surrounding tissues.
Although damaging excretions occur primarily in response to digestion of simple carbohydrates, plaque itself forms at a fairly constant rate, whether you are eating food or not. The growth rate of bacteria within plaque can be phenomenal. Under ideal conditions some types of bacteria can reproduce every 1 1/2 hour or so. There are 16 ninty minute periods in a 24 hour day. Doubling that bacterial population 16 times means a 65,536 fold increase (2 to the 16th power) in that population each day. That speaks to the reason dentists often suggest brushing at least three times daily, to keep the population of bacteria down to a reasonable level.
The bacteria which cause damage depend upon their plaque fortress to survive. So it is not necessary to actually remove the plaque from your tooth surfaces. Mechanically disturbing the plaque is enough to prevent bacteria from doing their dirty work.
Tooth decay occurs when bacteria within plaque are offered a diet of simple carbohydrates, or sugars. As the bacteria digest the sugar, they excrete acids which in turn demineralize and decay your teeth. The amount of damage is determined by factors such as the time your teeth are exposed to that acid, the strength of the acid, and the resistance of your tooth enamel.
The intensity of acid depends on the level of plaque on your teeth and the type of carbohydrates you are consuming. Bacteria digest simple carbohydrates (e.g. sugars) faster than they digest more complex carbohydrates (e.g. starches). The faster the bacteria digest their food the more intense the acid they produce. So simple carbohydrates are more damaging than complex carbohydrates.
The amount of time your teeth are exposed to acid depends on the length of time your teeth are exposed to sugary substance. Stickier foods (chewy candy) remain on your teeth longer than non-sticky substances (soft drinks), and are thus more harmful. In addition, bacteria continue to produce acid for 20 minutes or so after removal of sugar from their environment. So spreading the consumption of candy or soft drinks over a prolonged period of time exposes your teeth to acids for a longer period of time, and is thus more harmful dentally than consuming the same amount of candy or pop in one sitting.
The resistance of your tooth enamel can be influenced by the presence of fluoride and other environmental factors when your teeth were developing. Genetics also seems to play a role.
The acids and other irritants produced by bacteria within plaque affect not only tooth structure, but also the tissues surrounding and supporting your teeth. The irritants cause the gums to become swollen and red (gingivitis) and the bone supporting the tooth roots to resorb (periodontitis).
Gum diseases rarely hurt until they have progressed to a very late stage. Yet periodontitis, not tooth decay, is the most common cause of tooth loss in adults. That's why examination for gum disease is such an important part of regular dental checkups.
Malalignment of the teeth and mismatch of upper and lower jaws (malocclusion) can lead to functional problems, not just esthetic ones. Tooth crowding can cause difficulty cleaning, and it can lead to unfavorable bony contours around the tooth roots. This can predispose the teeth to decay and gum disease. It can also subject individual teeth to unusually heavy and uneven biting forces, resulting in accelerated bone loss or tooth fracture.
Some types of malocclusion can exert unhealthy pressures on the jaw joints, resulting in TMD (temporomandibular joint dysfunction).
The jaw joints are part of a tripod system among the teeth, and the right and left jaw joints (TMJ - temporomandibular joints). Imbalances in one part of that tripod affect the others.
Fragile muscles and ligaments support a small cartilage disk between a bone in the skull (articular eminence) and part of the jaw bone (condylar head). Normally the disk allows rotational movement as well as forward and backward translational movement as the jaw opens and closes. If those tiny muscles or ligaments become tense or stretched, the disk can slip in and out of the space between those bones during jaw movement. This can cause clicking or popping sensations. If the disk surface becomes damaged, a perforation or other irreversible change can occur.
TMJ problems are usually much easier to treat if detected early on in the disease process. Some late stage damage is just not treatable, and it can be debilitating. So it's important to have signs and symptoms checked regularly.