5 % ▁of ▁the ▁population ▁in ▁industrial ized ▁countries , ▁mainly ▁adult s ▁around ▁the ▁ages ▁of ▁ 5 0 ▁to ▁ 6 0 . ▁The ▁disease ▁is ▁now ▁decl ining ▁world - wide . ▁ ▁Pre val ence ▁of ▁Period ont al ▁D ise ases ▁in ▁Ad ult s ▁Many ▁studies ▁look ▁at ▁the ▁pre val ence ▁of ▁“ ad vanced ▁period ont itis ”, ▁but ▁have ▁differ ing ▁definitions ▁of ▁this ▁term . ▁ ▁Gener ally ▁though , ▁severe ▁forms ▁of ▁period ont itis ▁do ▁not ▁seem ▁to ▁affect ▁more ▁than ▁ 1 5 % ▁of ▁the ▁population ▁of ▁industrial ized ▁countries . ▁ ▁The ▁proportion ▁of ▁such ▁subjects ▁increases ▁with ▁age ▁and ▁seems ▁to ▁peak ▁between ▁ 5 0 ▁and ▁ 6 0 ▁years . ▁ ▁A ▁later ▁decl ine ▁in ▁pre val ence ▁may ▁be ▁due ▁to ▁to oth ▁loss . ▁ ▁There ▁are ▁a ▁number ▁of ▁method ological ▁concerns ▁with ▁pre val ence ▁studies , ▁particularly ▁ 1 ) ▁the ▁ability ▁of ▁partial ▁recording ▁to ▁reflect ▁full - mouth ▁conditions ▁and ▁ 2 ) ▁the ▁use ▁of ▁the ▁Community ▁period ont al ▁index ▁of ▁treatment ▁needs ▁( CP IT N ) ▁recording ▁system . ▁ ▁The ▁performance ▁of ▁a ▁partial ▁recording ▁system ▁is ▁affected ▁by ▁the ▁actual ▁pre val ence ▁of ▁period ont al ▁disease ▁in ▁the ▁population ▁in ▁question . ▁ ▁The ▁less ▁frequent ▁the ▁disease , ▁the ▁more ▁difficult ▁it ▁becomes ▁for ▁a ▁partial ▁recording ▁system ▁to ▁detect ▁it ▁and ▁thus ▁may ▁lead ▁to ▁greater ▁under