Categories of understanding along with everything else alter as societies change. - William Hardy McNeill

Before the eighteenth century the demographic impact of the profession of medicine remained negligible. Relatively few persons could afford to pay a doctor for his often very expensive services; and for every case in which the doctor's attendance really made a difference between life and death, there were other instances in which even the best available professional services made little difference to the course of the disease, or actually hindered recovery. ...Only with the eighteenth century did the situation begin to change; and it was not until after 1850 or so that the practice of medicine and the organization of medical services begin to make large-scale differences in human survival rates and population growth.

Ingenuity, knowledge, and organization alter but cannot cancel humanity's vulnerability to invasion by parasitic forms of life. Infectious disease which antedated the emergence of humankind will last as long as humanity itself, and will surely remain, as it has been hitherto, one of the fundamental parameters and determinants of human history.

The laws of nature, as analyzed mathematically and descriptively by Ptolemy and Galen, bore an interesting, and perhaps not entirely accidental similarity to the law of nations and of nature, as discerned by a long succession of Roman jurists. ...The concept of an objective law applicable to human affairs, yet operating in accord with Nature and Reason and apart both from divine revelation and from human whim or passion, was peculiar to Rome and societies descended from Rome.