One of the major effects of eating too much sugar is a high incidence of tooth decay. When we eat something with sugar in it, particularly refined sugar, enzymes in the saliva in the mouth begin to work immediately to change that sugar into a type of carbohydrate. ¹As one eats, particles of the sugary food get stuck between the teeth and around the gums. As the food changes its chemical composition, the resultant carbohydrate produces bacteria that begin to eat away at the enamel on the outside of our teeth. This is ²actually the decaying of the tooth. ³Now, if this process happens each time we eat sugar, we can see that eating excessive amounts of sugar causes more and more tooth decay. It is true that some tooth decay can be avoided with immediate brushing after eating, removing all the particles of food trapped in the teeth. ⁴However, sweets are often eaten as snacks between meals and during the day, times when people generally do not brush after eating. ⁵Therefore, the dangerous process of tooth decay is allowed to continue.

(Smalley, R. L. and Hank, M.R., REFINING COMPOSITION SKILLS. 1982, Macmillan Publishing Co., Inc., p.255.)

The word that DOESN'T have an irregular plural form like "tooth - teeth" is...