By examining the many new diseases that have appeared over the past two centuries, historians have categorized the ways in which diseases emerge. New causes (e.g., severe acute respiratory syndrome, motor vehicle accidents, radiation poisoning), new behaviors (cigarette smoking, intravenous drug use), and even the consequences of new therapies (insulin transforming the course and manifestations of diabetes) can produce new diseases. Changing environmental and social conditions can increase the prevalence of once-obscure ailments (myocardial infarction, lung cancer, kuru, and “mad cow” disease). New diagnostic technologies and therapeutic capacity can unmask previously unrecognized conditions (hypertension). New diagnostic criteria can expand a disease's boundaries (hypercholesterolemia, depression). Changing social mores can redefine what is or is not a disease (homosexuality, alcoholism, masturbation). New diseases can emerge as the result of conscious advocacy by interested parties (chronic fatigue syndrome, sick building syndrome). HIV–AIDS alone demonstrates many of these modes of emergence. The emergence, recognition, and impact of disease are never just a bioscientific process; the advent of a new disease always involves social, economic, and political processes that shape its epidemiology and influence our understanding and response.
Even as prevailing diseases have changed, health disparities have endured. Inequalities in health status have always existed, regardless of how health has been measured or populations defined. When Europeans arrived in the Americas, they witnessed stark disparities in the fates of European, American, and African populations. During the ravages of 19th-century industrialization, physicians grew familiar with health disparities between rich and poor. Health inequalities remain ubiquitous, not just among races and ethnic groups but also according to geography, sex, educational level, occupation, income, and other gradients of wealth and power.And there's lots more in this essay, which is accompanied by a very nice interactive graph showing the change over time in the major causes of death in the US. That graph alone is worth the trip to this online article.
If you'd like to experience a century of change in medical practice through literature (beyond the New England Journal of Medicine, that is), I encourage you to read The Bone Garden, by Tess Garritsen, who is a physician turned novelist.