A rotten tooth is caused by damage to the tooth structure that effects the enamel (hard coating that protects the outer layer of teeth) and continues to spread to layers in the teeth (dentine and pulp). This damage can continue to spread until there is nothing left of the inside of the tooth. Signs that tooth decay is running rampant in your mouth can be a warning to get your tooth taken care of before permanent damage is done.
Cavities are black, but just because you see a cavity doesn’t mean your tooth is rotten. If you do not take care of the cavity then eventually the plaque and bacteria that caused the cavity, causes the rest of the tooth to become black, but this is mainly because at that point, the root of the tooth is dead -- there is no actual enamel, dentin and pulp to keep the tooth alive.
As with any type of decay, a mouth that has decaying teeth in it will smell bad. When you smell something really bad in your house you look for a dead mouse or other critter. Death smells when decaying. Essentially your mouth is decaying and dying which is why there is an odor associated with rotten teeth.
Change in Tooth Size
As the enamel in your teeth erodes from the acid from bacteria in the plaque, you might notice your teeth changing in size. As the bacteria eats the dentin and pulp your teeth become thinner and subsequently smaller.
Teeth will Fall Out
This is probably the very last thing you will notice if you have rotting teeth. Your teeth will begin to fall out. Remember when your teeth turned black it was because the root of the tooth was dying. A dying root can only hang on for so long to a dead tooth until it finally just breaks away.