_The influence of past diseases_ is perhaps most conveniently considered in connexion with environment. As infection must necessarily impair the vitality of the cells of the body, either temporarily or permanently, it would be natural to expect that those who live to a great age would show a remarkably clean bill of health. This is often true. But it is at first sight rather disconcerting to find that nearly half of the 824 persons between 80 and 100 years of age analysed by Sir George Humphry had had severe illnesses at one time or another; many of these, however, were acute infections. It is noticeable that 85 per cent were free from rheumatism in the hands (selected as a convenient test in his collective investigation), which in the light of present opinion that arthritis is the result of a focal infection would appear to show that these old people are remarkably free from chronic disease. Long-continued infection or intoxication would be a far more potent factor than a transient illness in producing permanent change in the cells. A short acute infection would cause changes in the cells which might, like those of fatigue, be temporary and recoverable. Saundby’s[63] dictum may be accepted as fairly accurate, exception being made for peripheral mutilations, namely, that those only can expect to live to extreme old age who at the age of 60 possess bodies free from disease. But critical examination would no doubt show that focal infections become increasingly frequent as the years go on; this is particularly true as regards oral sepsis; after two years’ experience of 500 pensioners at the Royal Hospital, Chelsea, Thompson and Todd have never seen a tooth in a healthy condition there. It is noteworthy that dental disease seems to have become common with the advent of civilization, as judged by examination of ancient skulls, though the loss of teeth is an accompaniment of the bony changes in old age, and very old people seldom have more than a few teeth left. On the other hand, an acute illness is often the apparent starting-point of old age; there may be a long and imperfect convalescence due to persistence of infection, or deterioration may be due to auto- or hetero-suggestion (_vide_ p. 48). The influence of syphilis, whether congenital or acquired, is one of the most important in the prevention of healthy longevity; it not only directly disables and kills, especially in infancy and in the fifth decade, but it damages the vitality of the cells thus producing degeneration and premature senility, and favours secondary infections.