William McNeill:

Infectious diseases rank among the most powerful selective forces in human evolution. This book surveys the impact that diseases had on history. It is mostly wild speculation. He starts by assuming that thousands of years of communication had permitted an "homogenization of parasitic organisms" across Egypt, Mesopotamia and India by 500 BC but he provides no evidence that such communication existed and amounted to anything comparable to today's globalization. Sea voyages between the Mediterranean and China were non existent, and even between India and China they must have been very rare. Overland, infected people have time to die before they reached their destination,and very few cities were big and dense enough to provide pathogens an ideal ground to spread. Yes, there were Christian traders near Pondicherry in India, but they didn't travel back and forth every week like today's executives. Their journeys took months if not years. If trade was mostly local, and only a few traders ventured over long distances, presumably traveling in small groups and meeting their buyers and sellers in small towns or oases, it is debatable how fast and far infectious diseases could spread. Certainly over the course of 2,000 years viruses and bacteria could spread over long distances even if contacts were minimal, but it is not provable in which direction. McNeill's hypothesis is that exchange of goods always implied exchange of infections.

McNeill however makes you think. Today, so many European tourists get sick the moment they land in India or Peru because they have never been exposed before to their germs. But then how is it that Marco Polo traveled from Italy to China and then to other Asian regions and returned to Italy without any major disease? How is it that caravans could travel the Silk Road without being decimated by diseases? Years later Jared Diamond in "Guns, Germs, and Steel" (1997) came up with his "continental axes" theory (preferential spread along east/west axes), that Eurasians ended up with shared diseases and resistance to those diseases. But, again, this contrasts with today's experience that we actually are not well adapted at all to Chinese or Indian or Indonesian diseases.

It also makes us wonder what the effect of the European conquest of Asia in the 16th century was: starting around 1500 Europeans did travel extensively back and forth (traders, armies, settlers). It doesn't seem to have increased the number of epidemics. On the other hand, European germs did kill millions in the Americas.

Ancient "plagues" are mentioned in ancient scripts (as well as in the Bible, which is a relatively young text). McNeill mentions famous epidemics: Athens 630-429 BC (which probably started in Ethiopia), Rome 165 AD and 251 AD, Byzantium 542 AD. He speculates that these "plagues" could have caused a decline in population and state resources which helped the Arabs invade the Mediterranean. The plagues of the Mediterranean could also explain why the center of Christian civilization shifted from southern Europe to northern Europe. McNeill thinks that northern Europe experienced a population boom, and that the Viking expansion can be explained with a population explosion in Scandinavia. Food production did increase in northern Europe, although better agricultural techniques were certainly part of the reason. McNeill analyzes patterns of population growth and records of epidemics in both China and Japan, and the record in Japan looks particularly appalling after the first encounter with foreigners (Buddhist missionaries from Korea in 552). In 808 half the population of Japan perished due to a disease that may have been the plague that had ravaged China in 762-806. McNeill sees implications for the states and empires in these events. Unfortunately both the data and the speculations are hard to believe. Data are uncertain and unreliable. His speculations focus on germs and minimize the impact of wars, politics, social upheavals and economics. He constantly begins sentences with "It is possible that". One "possible" after the other leaves a probability close to zero percent. McNeill also used primitive germ theory to draw bold conclusions such as the bubonic plague probably originated from northeastern India or Africa.

His thoughts on the bubonic plague are more interesting. The plague (the real plague) is caused by a bacterium called Yersinia Pestis. McNeill doesn't find plagues in the European records after the year 900, but then the Black Death of the 14th century kills one third of Europeans. He conjectures that the plague originated in Yunnan, China. The Mongols, by conquering the vast landmass from China to Russia, indirectly fostered a trade route that went across nothern regions than the thousand-year-old Silk Road (steppes instead of oases). The Mongols attacked Yunnan in 1252. The Mongols ruled China from 1271 until 1368. In 1331 the plague spread in Mongol China. In 1346 it reached Crimea, also a Mongol dominion. From there the Italian traders shipped it (literally) to the Mediterranean, and from there it spread north. This is all disputed today, but it is interesting to think that it "could" have happened that way: new trade routes offer new routes for infectious diseases to spread.

Similarly, McNeill advances a theory about the plague that raged between 1894 and 1921 in different parts of the world. He thinks this one too originated in Yunnan. In 1855 China sent troops to quell a rebellion in Yunnan and McNeill speculates that these soldiers may have spread the disease around China until it surfaced in Hong Kong in 1894. Hong Kong was one of the main ports of the world. Steamships had started cruising the world in the 1870s and they helped spread the disease from China to India in 1896 (McNeill writes 1898) and Manchuria in 1910-11. McNeill speculates that in 1911 the Manchu who ruled China were no longer able to keep Chinese traders out of Manchuria and these Chinese immigrants spread the disease along the railway, built between 1898 and 1903 by the Russians before the Japanese invaded in 1905. Again, some of the facts are disputed today, but McNeill convincingly argues that a combination of trade, war, technology and migration can spread a disease that would otherwise remain local.

The chapter on the epidemiological impact of Europeans over the Americans is more reliable. There is still broad agreement among epidemiologists and historians about the impact of European infectious diseases on the original American population (the "Indians" of Columbus). Those population were devastated by a series of epidemics caused by the importation of germs into the American continent by the European colonists. The American populations had never been exposed to some of the deadliest Eurasian germs: bubonic plague, measles, smallpox, mumps, chickenpox, influenza, cholera, diphtheria, typhus, malaria, leprosy, and yellow fever. The "Indians" had no acquired immunity to these infectious diseases, i.e. their communities had not acquired herd immunity to Eurasian pathogens. McNeill accepts Alfred Crosby's theory that these epidemics brought down the American empires, not Spanish warfare. McNeill describes in vivid terms the devastation caused by Europeans onto the native population: at least 50% and up to 95% of the native American populations were decimated within the first 100–150 years following 1492. (Estimates of the pre-Columbian population varies wildly, but today most historians narrow the number to 50-100 million). Both Cortez and Pizarro with their small bands of soldiers were able to conquer large empires because their germs killed more than a big army would have. Jared Diamond's "Guns, Germs, and Steel" follows in this tradition. McNeill mentions a single missionary who in 1903 unwillingly exterminated an entire tribe by exposing them to European pathogens. This may have been the fate of millions of Amerindians who were "converted" by missionaries. The big book on covid has not big written yet, but covid too seems to follow a roughly similar trajectory: Native Americans of the USA were impacted almost ten times more than whites of European descent, and the four countries with the highest death rates are all in America (Peru, USA, Chile, Brazil). At one point in 2022 the two countries with the highest death rate were Peru and Mexico, i.e. the descendants of the Aztec and Inca empires; an amazing coincidence. McNeill speculates that Amerindians relied more on agriculture than meat and their domestic animals were both rare and somehow didn't carry parasites that easily transfer to humans. Smallpox reached Hispaniola in 1518 (one year before Hernan Cortez sailed from Cuba to Mexico), reaching the Inca empire in 1525 (seven years before Fracisco Pizarro began his conquest of the Inca empire). The variola virus decimated the populations and even killed the elites (both the Inca emperor and his heir, causing a civil war that Pizarro then exploited). Some historians estimate that the smallpox epidemic of 1519-20 killed 5 million to 8 million people in Mexico alone. In 1531 it was the turn of measles, which decimated the population of Mexico. Surprisingly there were also non-Eurasian diseases that killed many more Amerindians than Europeans: the 1545-48 epidemic of "cocoliztli" killed an estimated 5 million to 15 million people, i.e. about 80% of the Amerindian population of Mexico, and the subsequent "cocoliztli" epidemic of 1576-78 killed an additional 2 to 2.5 million people, i.e. about half of the remaining Mexican Amerindians. This disease doesn't seem to be known in Europe (based on the symptoms reported by 16th century Spaniards and based on the fact that they spread mostly inland and not in the coastal areas colonized by the Europeans). McNeill speculates that it was typhus, but there is no evidence. It is instead proven that in 1557 the influenza pandemic originating from the Ottoman Empire spread quickly from Europe to Spanish America, brought by Spanish sailors, and in 1559 to Portuguese Brazil (where the missionaries are the more likely culprit). This may have been the first worldwide pandemic. The Amerindians of what is now New England in the USA were exterminated in 1616-19 by "The Great Dying", a disease that may have been smallpox, with the highest rate of fatalities concentrated around Boston Harbor and Plymouth Bay. When the English pilgrims of the Mayflower arrived in Plymouth Harbor at the end of 1620, they came to believe that God had wiped out the pagans on purpose so that Christians could settle there. Smallpox was certainly brought there in 1633 after hundreds of English families with children joined the early settlers. That epidemic all but finished the remaining population of local Amerindians. Then came the African slave trade and the African diseases: malaria and yellow fever. One hypothesis is that malaria spread from the African rain forest via the Nile to Egypt and the Mediterranean, where the Greeks picked it up and spread it to Italy, and from there the Romans spread it to the rest of Europe. (Some historians even speculate that malaria contributed to the fall of the Roman Empire). In around 1560 African slaves brought malaria's plasmodium parasite to Brazil, where tropical conditions were just perfect for its carrier, the mosquitoes. (The malaria mosquito, Anopheles, was probably already there). By 1750, malaria was common from Brazil to New England. Yellow fever reached Barbados in the Caribbeans in 1647 where Europeans had started sugar cultivation and started importing African slaves (in 1644 its population was about 30,000 English colonists and about 800 African slaves) and then spread to Mexico in 1648. Brazil's first recorded outbreak of yellow fever happened in 1685-90, and African slaves are again suspected as the human carriers. The ships carrying African slaves also brought the yellow-fever mosquito Aedes Aegypti. The effect of these African diseases was to replace Amerindian populations with African populations. (McNeill also mentions tuberculosis as a disease brought by Europeans to America but it is now proven that the disease existed long before Columbus' journey and it is likely that seals and sea lions are the ones that brought the disease to South America, although it is possible that the Amerindians were vulnerable to more virulent European strains of the disease).

There is one major disease that may have traveled the other way around. No medieval medical treatise mentions syphilis. Syphilis supposedly arrived with Columbus' crew. Some of them joined a French expedition into Italy in 1494-95, and then spread the disease to France. In 1498 it surfaced in India, and then in China and Japan. Syphilis is the first disease discovered after the invention of printing, which means that it was the first disease to make news all over Europe.

McNeill also mentions typhus as an "African" disease, but it was known in Spain in 1490, two years before Columbus' voyage, so it could be yet another disease that Europeans brought to America.

McNeill doesn't discuss one interesting aspect: when the first humans entered the American continent is still unknown, although generally believed to be about 13,000 years ago via the Bering Land Bridge (Beringia) when sea levels were very low, but human presence in the American continent is well established by the close of the Pleistocene, approximately 10,000 years ago. The Eurasians who originally explored and settled the continent apparently had no experience with all those Eurasian diseases who have been so widespread in Eurasia in the last two thousand years. This means that the two branches of humankind must have separated before those germs emerged, which means that those germs are relatively young. Somehow Eurasia produced a variety of deadly germs whereas America didn't.

The last chapter is a brief introduction to vaccination. Variolation (smallpox inoculation) was well known in the Ottoman Empire as a protection against smallpox (whether the Ottomans learned it from the Chinese or more likely from the Africans). In 1713 and 1715 respectively two ethnic Greek doctors of Istanbul, Emmanuel Timonis and Iakobos Pylarinos (known in Italy as Giacomo Pylarini), independently wrote about it the Royal Society of London. In 1718 Lady Mary Wortley Montagu, who was living in Istanbul (her husband was the British ambassador), "variolated" her child. In 1721 she variolated her daughter in London, the first person to be variolated in Europe. At the same time, also in 1721, Cotton Mather, a Boston Puritan (who had participated in the Salem Witch Trials), who had already learned of variolation from one of his African slaves, convinced a doctor, Zabdiel Boylston, to try it when a smallpox epidemic started spreading in Boston. Boylston inoculated his 13-year-old son, the first person to be variolated in America. The treatment was obviously effective, but Christian zealots claimed that smallpox was God's punishment for human sins and objected to interfering with God's will. This argument failed in Britain and the USA, but was somehow successful in continental Europe. In 1754 variolation was formally approved in Britain by the Royal College of Physicians. The widespread use of variolation helped population grow faster in Britain than in France. The first major non-British European to accept variolation was actually the Russian empress Ekaterina who hired a British physician in 1768. During the war of independence, George Washington mandated inoculation for all Continental soldiers in 1777. In 1796 a British scientist, Edward Jenner, developed the first smallpox vaccine (using the much milder cowpox) and this time France was at the vanguard to use it, especially after Napoleon mandated it to all his troops across Europe in 1805. During the 1860s Robert Sutton, who was not trained as a scientist, improved variolation making a lot more effective and safe.

McNeill then turns his attention to cholera, whose initial spread he blames on Hindu pilgrimage (the Kumbh Mela of 1817) and Muslim pilgrimage (the Haji of 1831). Until the 18th century the most lethal pandemics in Europe were caused by bubonic plague. Cholera became the major disease of the 19th century and the first one for which the worldwide spread could easily be documented (because most of the world was ruled by European empires and because of the rise of the newspaper). The whole century is littered with cholera epidemics in all continents, starting with the Indian epidemic of 1817.

McNeill briefly dwells on the rise of modern medicine. For centuries two theories about diseases existed: Galen's miasma theory, which diseases are caused by "miasmas" in filthy spaces (basically, by poor hygienic conditions) and the germ theory. The former prevailed. The germ theory had been originally advanced in 1546 by the Italian anatomist Girolamo Fracastoro, whose book is the first book to explain how physical person-to-person contagion spreads a disease, and then perfected in 1762 by the Austrian physician Marcus Antonius von Plenciz. In 1835 the Italian entomologist Agostino Bassi discovered that a microorganism is responsible for spreading a silkworm disease. Contagionists, mainly in Italy and France, believed in quarantines, a practice inaugurated in Venezia/Venice in 1374. Anticontagionists accused contagionists of being superstitious and in any event quarantines were hated by the general population. Distinguished scientists sided with the anticontagionists after the first scientific studies of epidemics: Benjamin Rush became anticontagionist after the yellow-fever epidemic of 1793 in Philadelphia, which led to the founding in 1799 of the anticontagionist Philadelphia Academy of Medicine, in 1802 Alexander von Humboldt sides with the anticontagionists, and the French scientists got convinced when in 1802 French troops were annihilated by yellow fever in Haiti. Another famous epidemic of yellow fever, in 1819-21 in Spain, led both the British physician Robert Jackson ("Observations of the Yellow Fever in Spain", 1821) and French physician Nicolas Chervin in 1825 to conclude that yellow fever was not caused by contagion. In 1828 the French Academy of Science (the leading medical institution of the era) sided with Chervin. Britain thus proceeded to improve the sanitary conditions of cities, famously after Edwin Chadwick's report "Report on The Sanitary Condition of the Labouring Population of Great Britain" (1842) which led to the Public Health Act of 1848 and the appointment of John Simon as Medical Officer of Health for London. At last, in 1854 the British physician John Snow proved that the London cholera outbreak of 1849 could be traced back to a single contamination with human feces of the drinking water supply. In the same year, 1854, the Italian anatomist Filippo Pacini isolated the cholera bacterium Vibrio Cholerae, but the discovery was ignored for decades. (McNeill doesn't mention him, but in 1850 the French physician Casimir Davaine had already identified the bacterium that caused anthrax, "Inoculation du sang de rate", 1850, and this was the first association of a microorganism with a disease). In the 1870s the French chemist Louis Pasteur accepted the germ theory of disease. In 1879, using a microscope (invented in the 17th century by Leeuwenhoek), German bacteriologist Robert Koch identified the bacillus that causes anthrax and in 1881 Louis Pasteur developed a vaccine for anthrax. They proved beyond any doubt the germ theory of disease. Koch identified the bacillus that causes tuberculosis in 1882 and in 1883 rediscovered Vibrio Cholerae. In 1885 Pasteur developed a vaccine for rabies by attenuating the virus in rabbits (he didn't know that it was a virus, just that it was a germ). In 1896 Almroth Edward Wright, Richard Pfeiffer and Wilhelm Kolle developed the typhoid vaccine.

McNeill, who generally omits the history of viruses, doesn't mention it, but the first discovered "human" virus was the virus of yellow fever. In 1897 British physician Ronald Ross proved that malaria is spread by mosquitoes. In 1881 the Cuban physician Carlos Finlay had advanced the theory that yellow fever was spread by mosquitoes. In 1900 a US team led by Walter Reed, inspired by Ross' experiment, proved that Finlay was right (Cuba at the time was administered by the USA). The virus itself was not isolated until 1927 and a vaccine was available only in 1938).

Until the 19th century, it was common to lose a child to disease.