There are a lot of diseases in the world. With tens of thousands of diseases out there, it can be tricky for a storyteller to know which ones are actually likely to show up in a story. As it happens, we know a lot about which diseases are the deadliest and most debilitating both in the high-income countries where you probably live and the low-income countries which likely approximate your campaign setting.

There are a lot of diseases in the world. Depending on whose statistics you read, modern medicine identifies anywhere from around 7,000 to 20,000 different medical problems, the wide variety being due to, for example, some sources counting only disorders with a genetic contribution, versus other sources who might count different types of knee dislocations as individual problems. The vast majority of these disorders are incredibly rare. OprhaNet, a organization dedicated specifically to rare and under-recognized (or "orphan") diseases, published their updated list of over 6,000 rare diseases this past November, hundreds of which have been reliably documented less than 10 times in history. The most likely explanation for the rarity of these diseases is that for most of history, nobody was looking for them, and unless you have pretty sophisticated laboratory tests and imaging and an extremely obsessional personality, then, for example, Pollitt syndrome (10 recorded cases) may not be easily differentiated from any other form of developmental delay combined with various physical manifestations. With tens of thousands of diseases out there, it can be tricky for a storyteller to know which ones are actually likely to show up in a story, especially since your average storyteller doesn't have a PhD in epidemiology.

As it happens, the World Health Organization maintains a nifty database that can be of help with this question. They routinely publish data not only on the most prevalent, debilitating, and deadly diseases in the world, they also provide separate data for high-income and low-income countries. The data on high-income countries is more likely to be of personal interest (if you're reading this, you presumably live in a high-income country, after all), whereas the low-income data, in addition to hopefully stimulating your social conscience, likely reflects, to some degree, the distribution of disease in a medieval fantasy setting.

In 2011, the ten deadliest medical conditions in the world were heart disease, stroke, lower respiratory infections, chronic obstructive lung disease, diarrhea, HIV, lung cancer, diabetes, road accidents (it's debatable whether this is a "disease" or more of a mechanism of injury, but you get the idea), and disorders associated with premature birth. We see some very interesting differences between low- and high-income countries. In highest-income countries, the ten biggest causes of death were ischemic ("not enough oxygen") heart disease, stroke, lung cancer, Alzheimer's and other dementias, chronic obstructive lung disease, lower respiratory infections, intestinal cancer, diabetes, hypertensive heart disease, and breast cancer. In lowest-income countries, the list goes, lower respiratory infections, HIV, diarrhea, stroke, ischemic heart disease, prematurity, malaria, tuberculosis, malnutrition, and trauma during birth. The differences are pretty striking. Lower respiratory infections -- pneumonia, for example -- remains one of the main killer, because it's the end-point of a lot of other diseases. Ischemic heart disease remains on the list, but is much less of a killer because people don't eat such high-fat diets, get more exercise, and by and large die of something else first. Similarly, people don't die so much of cancer in poor countries, and Alzheimer's isn't on the list, because to die of something like that you have to live a relatively healthy life and reach a relatively advanced age. With malaria and HIV killing off large swaths of the population before the age of 50, you can imagine how heart attacks, possibly the single most feared medical condition in the Western world, might not even be on the radar of someone living as a subsistence-farmer in rural Africa. The WHO notes that in high-income countries, 7 in every 10 deaths happen after the age of 70, whereas in low-income countries, 4 in every 10 deaths are before the age of 15 and only about 2 in 10 people live long enough to die after 70. You can probably imagine which set of numbers more closely approximates the world that your player characters come from.

There's another way to look at this data, though, which is in terms of disease burden. Not every disease kills you right away; many of them can first rob you of a few decades of productive work and play. The WHO uses a calculation called the DALY, or disability-adjusted life year, which is essentially a measure of years of productive life that different diseases steal from a person both in terms of YLL, or Years of Life Lost, and the equally scary YLD, or Years Lived with Disability. Below, I've reprinted the WHO's tables comparing years lived with disability from lowest- and highest-income countries to allow you to draw your own conclusions. "YLDs per 100,000 population" means, for every 100,000 people in a population, this many years are lived with disability.

More than four years ago, Dr. Eris Lis, M.D., began writing a series of brilliant and informative posts on RPGs through the eyes of a medical professional, and this is the one that appeared here on March 8, 2014. Lis is a physician, gamer, and author of the Skirmisher Publishing LLC OGL sourcebook Insults & Injuries, which is also available for the Pathfinder RPG system.