Parmly (1819) proposed that an unidentified “chymical agent” was responsible for caries. According to this theory, teeth are destroyed by the acids formed in the oral cavity by the putrefaction of protein which produced ammonia and was subsequently oxidized to nitric acid. Robertson (1895) proposed that dental decay was caused by acids formed by fermentation of food particles around teeth.
Parasitic or septic theory.- stated that microorganisms can have toxic and destructive effects on dental tissues. Ficinus ,a dental physician implied that filamentous microorganisms (denticolae) in the enamel cuticle and in carious lesions caused decomposition of the enamel and dentin.