Final answer:
Diabetes is included in the Armitage 1999 classification of periodontitis as a manifestation of systemic disease, while atherosclerosis and osteoporosis are not.
Step-by-step explanation:
Periodontitis as a manifestation of systemic disease, as described in the Armitage 1999 classification, includes conditions that are associated with systemic diseases that can also affect the periodontium. Diabetes, for example, is traditionally categorized as a metabolic disease, which begins as chronic inflammation and is now classified by many scientists as an autoimmune disease. Other conditions that were found to be related include leukemia and genetic disorders; however, atherosclerosis and osteoporosis are not traditionally included in this classification.