Plagues and Peoples (William H. McNeill, 1976)

Relatively short, but quite interesting throughout.  I understand that McNeill's thinking was rather groundbreaking in the mid-70s; I've seen elements discussed in this recent (and useful) book.

His thesis:  that the role of disease has been under-appreciated when assessing historical trends.

Another main theme:  applying the notion that microscopic parasites - disease - optimize when achieving a sustainable balance with hosts.  A disease that kills off its hosts too vigorously isn't going to thrive.  When disease exposure occurs regularly among a sufficiently large population, the disease typically turns into a childhood disease - killing off some of the weak - while survivors are immune (good for microparasite and host alike).  Isolated populations are at great risk - when exposure to a new disease finally occurs, carnage among adults occurs - a much more grievous blow to the community.

It isn't so much that those themes are surprising (though that may have been the case to some extent when originally published) - the value is his application of the themes (admittedly speculative in plenty of cases) to various historical episodes.

The big example of course (also addressed at length in 1493) is the die-off in North America resulting from the Columbian Exchange.  But McNeill provides plenty of other interesting examples from around the world.

I like how he links the microparasites to what he refers to macroparasites - almost exclusively humans preying on other humans.  The same basic principle applies - if the conqueror is too savage - or demands too much in rent or taxes - the conquered population withers and isn't productive, to the conqueror's disadvantage.

Many times the macroparasites (armies) were the instrument via which microparasites were transmitted to isolated populations, to lethal effect.

More microparasites thrive in hot climates.

City-dwellers died off due to unsanitary conditions, but were less prone to die-off from diseases (large population, steady exposure to microparasites).

Shipping, caravans, armies spread diseases in olden times.  Widespread travel in 19th and 20th centuries results in fewer epidemics - most microparasites are distributed everywhere relatively quickly.

McNeill links illness-induced weakness to various conquests and power shifts.

Rise of modern medicine has put microparasites on the defensive and allowed unprecedent population surges (along with better agriculture, etc.)  McNeill expects the microparasites to keep adjusting.