1) the etiologic agents must come from outside the host organism through the portal of entry some time prior to development of symptoms.

2) the must be present in sufficient numbers to initiate disease activity (infectious dose).

3) the host must be in a susceptible state for the infection to develop.

Infectious disease conditions for periodontitis are met in the following ways:
Pathogens are transmitted from one mouth to another largely by the acquisition (inoculation) of saliva.

Caries pathogens such as Streptococcus mutans are transmitted vertically from mother to child through saliva transfer. There is evidence for horizontal transmission of anaerobes between spouses.

If biochemical conditions are favorable, these bacteria introduced through saliva transfer can develop sufficient numbers to colonize and lead to an infection.