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The Infectious Disease Model is
widely accepted in medicine and elucidated in most Medical Microbiology
textbooks. 9 Some important concepts in this model are "portals of
entry" and "infectious dose". Portals of entry are pathways by
which the infectious agents enter the body. The infectious agents may be viruses
(HIV, HBV), bacteria, fungi, parasites, etc. The infectious dose is that number
of pathogens needed to initiate an infectious process. 10
In periodontitis, the portal of entry is the mouth and saliva. The sites of infection are the root surfaces, periradicular pockets and the periodontium itself. Since there is a complex host-bacteria interaction involving the immune system and other factors, the infectious dose needed to start periodontal infections will vary between patients.
When an infection becomes acutely active, inflammation may be detected in a dental office or by patients in the form of gingivitis, periodontal abscess or loose teeth in severe cases.