Bacteria were more powerful than emperors

This is the popular version, good for the Europeans’ feeling of superiority, but not completely true. Montezuma and his people did indeed initially see the Spaniards as gods, but soon knew better and discovered that horses are very vulnerable and that the primitive rifles were not too dangerous. It is true that Cortez enlisted the help of Montezuma’s enemies, but the Aztec empire was mighty and strong.

It remains amazing that the native religions of the peoples of America so quickly made way for Christianity. For a long time Europeans explained this as due to the superiority of their civilisation or to heavenly guidance. But no miracle of God was necessary here, merely a minuscule microorganism.

A Flemish version of the sailing ship with which the marinheiros changed the world. (From:: Graphic Worlds of Peter Bruegel the Elder)

During the night in which the Aztecs successfully expelled Cortez and his men, a smallpox epidemic broke out in Mexico; many soldiers died. De defenders of the city thus could not pursue the Spaniards and the latter soon returned as victors to the city. Probably the Spanish success was mainly caused by a psychological effect: the Spaniards did not become ill, thus they must have heavenly protection, so thought the Aztecs. Their god must indeed be very powerful. (In reality the Spaniards had been infected as youths and had thus acquired immunity against the smallpox virus).

The inhabitants of the New World had until the arrival of the Europeans, very little bother with disease, but because of this also no resistance to diseases of the newcomers from across the ocean. History repeated itself many times with various diseases and each time this lesson could be learned: the god of the Christians is more powerful. Much better to honour him.

Nezahualpilli (1460 - 1515), king of Texcoco when the Spaniards conquered Mexico. From: Bibliothèque Nationale in Paris.

The cultural attainment of the civilisations of the Aztecs and the Incas was well comparable to that of Europe at that time. The cultural superiority of the Europeans cannot explain the speedy conquest of America. The fact that a whole range of diseases had occurred in Europe, against which adults were mostly immune and the people in the Americas were not, can explain this. The Indians had to reach the stage in their struggle against diseases, in a few decades, which the Europeans had reached in a few thousand years.

The Black Death

Patients suffering from smallpox (click on the picture

The story of the Black Death in the fourteenth century is generally known, but historians have never had much interest in the influence it had on human history. Now we know more, this influence turns out to have been very large.

Because very big epidemics have not occurred for the last two centuries historians in the nineteenth and twentieth centuries often regarded stories in old documents as exaggerations - but they were not. Our ancestors in earlier centuries were involved with ever new diseases against which no-one was immune and again and again equilibrium had to be restored, at the cost of many deaths. A stable relationship between host and parasite means that both remain alife and propagate themselves. This implies that the parasite had better not kill its host too quickly. A living host gives it the chance to spread itself.

The spread of the Black Death in Europe between December 1347 and December 1350. By the end of 1347 the disease had Turkey and areas around the Black Sea as well as Sicily, Sardinia and Marseilles in its grip. Three years later the black death had invaded just about the whole of Europe. Only Finland and part of Scotland were still free of it.

A new parasite often cannot accomplish this, it kills its host and disappears. This may have happened often, but was seldom confirmed: some people would die as a result of an unknown disease and the latter then disappeared - as recently happened with Lassa fever in Nigeria and the O’nyong fever in Uganda.

A parasite able to maintain itself, becomes less virulent. We see this today with AIDS. Initially people died very quickly from it; now chances of survival are quite good - apparantly not only from medicines - and there are people who have been seropositive for a long time but still not ill.

The common cold is therefore undoubtedly a long known ‘oldie’ of humans from before the start of history!

Before the beginning of history

It may appear farfetched to discuss military systems, but we shall see that at times diseases and armies – or micro and macroparasites – had comparable and reinforcing effects on populations.

Prehistory

Early man presumably had to contend with a large number of parasites, such as we still see with the different types of apes. Humans lived after all in the tropics swarming with parasites (we know of about twenty types of malaria in primates, of which four are now found in humans).  For as long as our ancestors lived in the rainforests there may have been a reasonable balance with the different parasites. The whole rainforest represents an ecosystem which has evolved in 60 million years to a completely balanced system. The parasites may thus have been not too virulent and the primates lived in reasonable health.

When man with his greater intelligence and his language began a sort of accelerated evolution (a cultural evolution as it were on top of the biological evolution) disease patterns must also have changed.

Humans have developed so fast since that time that a balanced situation never could develop.

Real modern humans of our type have existed for a few hundred thousand years at most, but only within the last few thousand years have different civilisations developed. For most of the time ( for us a very long time but in evolutionary view  a short time) humans lived in small groups without fixed habitation, hunting animals and collecting fruits and roots, in tropical Africa. These people undoubtedly were bothered by worms and other tropical parasites, but droughts, grass fires and other problems also kept the population small. Humans were a rare species.

About 50,000 to 40,000 years ago (perhaps after the invention of clothing) humans also began to inhabit cooler climates. They appeared in Europe, Asia, America, Australia, i.e. everywhere except in Antarctica.

This means adapting to colder climates, but it was especially a liberation from tropical diseases. Moreover no other primates lived in these cool regions from which humans could take over diseases. The lack of diseases caused a population explosion, a human plague. Such a plague usually disappears of its own accord, but we people are capable of opposing this using our cultural adaptation (whether this will also be apparent on an evolutionary scale remains to be seen).

In any case many types of large mammals died out at that time, almost certainly through excessive hunting. The biodiversity decreases wherever humans appear – this is still occurring and the rate is increasing...

For a long time we therefore had two systems: in the tropics the human population kept in check by all sorts of illnesses and parasites; in the temperate and cold climates this was done by the presence or absence of wild animals and fish. The hunter- gatherers in temperate climates were presumably seldom ill. This is today still the case with tribes having no contact with the modern world, living a so-called primitive livelihood. Parasites are not able to spread because these humans live in small groups without much contact with others.

The start of history

Through the extinction of the large wild animals humans had to find another niche and agriculture and animal husbandry became necessary and through this the future was open to diseases and plagues. The emergence of agriculture once again brought a population explosion and the start of an eternal struggle against certain sorts of plants (weeds) and animals (vermin) which threatened harvests.

People with fixed habitats have a greater likelihood of parasites, but for a long time the communities were too small for major problems. In areas with marshy agriculture worm infections will have become common (this has been confirmed in investigations of mummies). Because of these infections farmers were tardy and slow and an easy prey for the macroparasite par excellence: soldiers. Possibly the reduced resistance of farmers is an explanation for the development of despotic empires.

Until about one hundred years ago we knew nothing about the real cause of diseases. Yet we observe a regimen arising in various civilisations which limited the spread of diseases: ritual washing in various religions, isolating lepers, banning the consumption of pork with Jews and Moslems (pigs consume everything including corpses and excreta, through which they can carry various diseases, including tape-worm). It is likely these taboos have have arisen first  from disgust, but people learned though experience that they made sense.

The start of history

Through the extinction of the large wild animals humans had to find another niche and agriculture and animal husbandry became necessary and through this the future was open to diseases and plagues. The emergence of agriculture once again brought a population explosion and the start of an eternal struggle against certain sorts of plants (weeds) and animals (vermin) which threatened harvests.

People with fixed habitats have a greater likelihood of parasites, but for a long time the communities were too small for major problems. In areas with marshy agriculture worm infections will have become common (this has been confirmed in investigations of mummies). Because of these infections farmers were tardy and slow and an easy prey for the macroparasite par excellence: soldiers. Possibly the reduced resistance of farmers is an explanation for the development of despotic empires.

Until about one hundred years ago we knew nothing about the real cause of diseases. Yet we observe a regimen arising in various civilisations which limited the spread of diseases: ritual washing in various religions, isolating lepers, banning the consumption of pork with Jews and Moslems (pigs consume everything including corpses and excreta, through which they can carry various diseases, including tape-worm). It is likely these taboos have have arisen first from disgust, but people learned though experience that they made sense.

On the other hand areas where native parasite cause serious illnesses (malaria, sleeping sickness) were never developed to real agricultural areas. In the past century attempts to colonise such areas failed (in Central Africa). This is likely to be the most important reason why a large part of Africa has fallen behind in development.

Once human communities became larger and travel between them more intensive, diseases which could reach new hosts without the mediation of mosquitoes and the like, got their chance. These create immunity and must therefore ensure that they can be quickly carried over to another person. A person who is infected dies soon or becomes immune in a few weeks.

Infectious diseases can only survive in reasonably large communities. Otherwise at a certain moment everyone is immune; before this should happen, a disease can only survive if there is a new generation of children present. Only communities of thousands of people, who are also living closely together so that the disease can be carried over continually, can offer a chance of survival to bacteria and viruses, which proceed directly from person to person. In other words: illnesses such as measles, smallpox and whooping-cough are bound to what we call civilisation, with cities and surrounding areas with regular mutual contact. These illnesses currently occur all over the world (in so far they have not in the mean time been prevented by vaccination).

Civilised people

Research shows that measles keeps on recurring only if at least 7,000 susceptible persons constantly are available; they are usually children who have just started school. A population of 300,000 to 400,000 persons appears to be the minimum within which the illness keeps circulating. (In Iceland an epidemic appears periodically although the local population is too small, but whenever there is a new generation of children, it returns with a traveller).

It is interesting that the oldest known urban civilisation consisted of approximately that number of people (the Sumerians). We may therefore also accept that “civilised” illnesses started their advance from approximately 3,000 BC.

Civilisations were centred in cities, but in the cities illnesses ruled. Until quite recently the number of deaths in the cities was so high that an urban population had to be replenished from the surrounding rural areas continuously. The cities thus not only needed the farmers for their food, but also for their demography - also because wars and sieges often claimed a high toll of the urban population (here again we have micro- and macroparasites in an equivalent role). That an urban population was sometimes replaced by rural people after an epidemic is also visible in language developments. In Mesopotamia the lingua franca Sumerian was in a short time replaced by Semitic; the original language remained as the language of science and religion. Something similar occurred in the Habsburg empire: Czech and Hungarian speaking rural people had always been migrating to the cities, but once there they learned German and after one generation they were indistinguishable from the other people - until in the 19^th century this migration became so strong that they kept on speaking their own languages and Prague became a Czech city and Budapest Hungarian.

Migration from rural areas to the city is thus as old as civilisation and always kept going because of diseases in the city and surplus births on the land.

Another side to this matter is that farmers’ sons were often less immune and in the armies and after migration to the cities more often died of illnesses for which the city people were immune.

The origin of disease

M ost infectious diseases have been taken over from our domesticated animals: measles is probably related to rinderpest or distemper, smallpox from cowpox, flu from pigs, and so on. Wild animals also brought one and the other: bubonic plague comes from rats, yellow fever from apes and rabies from bats.

Undoubtedly diseases often arose which either eliminated the whole local population, or caused immunity so quickly that they just as quickly disappeared. Recent cases are known.

A number of bacteria and viruses have actually appeared to be equally successful as their host, the human.

(The parallel with the ‘human parasite’ covers quite a lot: only when there were fairly large prosperous cities did it become worth while to conquer them. A community that is large enough becomes immune, i.e. can raise its own army to keep the parasites outside the borders. Such an army costs food, but keeping an army is safer than doing nothing and waiting for the enemy).

Our cattle are descended from species which wandered the steppes and undoubtedly harboured various diseases. They had themselves through a long evolution developed an equilibrium, so that they no longer became ill. In such animals only weak symptoms occur, but when these diseases pass over to humans they are initially very virulent. Many died, but the surviving persons were immune and gradually the disease became less severe and usually became a children’s illness.

The history of diseases

Old documents from China, Egypt, Babylon as well as the Old Testament mention terrible epidemics (think of the plagues of Egypt). Often the diseases are called “plague”, but these could have been all sorts of diseases which cause enormous numbers of deaths in a short period (measles, smallpox, influenza, typhus and so on). The density of the population was large enough to make epidemics possible; there must have been a certain equilibrium as some empires (Persia, Assyria) held on for centuries.

The civilisations which developed around the Mediterranean Sea presumably had relatively little bother from diseases and plagues for a long time: the most important crops - the olive and the vine - were descended from local species and had few diseases and people there had no marshy agriculture. The most important grains were also derived wild species from neighbouring areas (the Middle East) and chiefly imported from surrounding “barbarian” lands. But with the growth of the population centres and trade the number of diseases increased.

Hippocrates describes a number of diseases accurately enough to be able to conclude which they were: malaria and mumps occurred in his time, but no tuberculosis, diphtheria or influenza.

The populations grew and colonised the area around the Mediterranean Sea and just beyond it (at first the Greeks, later Carthage and Rome). Shipping ensured mutual contact and was fast enough to bring diseases as well. The whole Mediterranean area was one ‘pool’ as far as diseases were concerned.

At the beginning of our era the Roman and the Chinese empires and possibly the Indian empire were approximately equally big; between 50 and 60 million people.

They had mutual contacts but regular caravan traffic only began a few centuries later and with that also an exchange of diseases. At this time each region had its own pattern of diseases and epidemics, determined mostly by climate and agricultural methods.

Those of the Roman Empire are the best known. Livius describes eleven large epidemics. We also know of an epidemic in 165 AD which the soldiers brought with them from Mesopotamia and which lasted fifteen years. This was clearly a new infection for this region and the number of deaths was enormous: one quarter to one third of the population died in the affected areas. We are not certain which disease was involved; possibly smallpox or a precursor. This epidemic also started a general decrease of the population through a series of epidemics in the Roman regions.

This was probably Europe’s first acquaintance with two illnesses which later became normal childhood diseases in Western Europe: measles and smallpox. The descriptions are somewhat vague, partly because people until the 16^th century made no distinction between these diseases.

The development of a number of diseases to some sort of equilibrium has followed a fixed pattern: the cause usually came from one of the domesticated animals or animals living in the vicinity of humans such as rats when they at first come into contact with the humans. Those people have no resistance at all and therefore become seriously ill (probably many diseases destroyed themselves when the whole infected population died). The survivors were immune and when the infection returned after a number of years, the children became ill, but not the adults. Moreover an illness gradually appears to become less virulent. From history it appears that it sometimes took several generations (and epidemics) before this happened, for example if the time between epidemics lasted longer than one generation.

The end of the Romans and the beginning of the Christian era

B ack to the Romans: a series of severe epidemics in the first centuries of our era killed large parts of the urban population around the Mediterranean Sea. As a result much less taxation was received and the soldiers went unpaid. They started mutinying and plundering and in passing spread the disease over the rural areas, causing food production to decrease and the resistance of the people to become even worse. For some centuries the relationship of the land with the micro parasitical upper class was reasonably tolerable; but through the combined micro and macroparasitical attacks in these centuries conditions rapidly deteriorated. The role of diseases in the fall of the Roman empire, the great migration of the nations and the like have for a long time been underestimated, because historians did not realise that illnesses such as measles were then not the innocent childhood illnesses we now know, but caused deadly epidemics which completely dislocated society. In reality Europe then experienced the same process that the Amerindians had to undergo quickly after Columbus.

In the mean time Christianity arose, probably partly due to successive epidemics: Christians helped to care for their fellow humans even in times of pestilence, causing more of them to survive and strengthening their relationship. The Christians did also not despair as much, because for them death meant passing on to eternal life. Christianity therefore meant a good adjustment to withstand such times and as a result won many followers.

At the same time the plague must have invaded Europe for the first time, brought by the black rat, an animal formerly found in India as a wild animal, which had learned to survive in a human environment and could be easily spread by ship, at first to the harbour cities around the Mediterranean Sea. Rats do not always carry the plague bacillus but can easily exchange it with rodents which do not approach human environment too closely. These animals later on built up a reservoir of plague bacilla on the Mongolian steppes and much later also on the American prairies from where more epidemics arose.

Diseases in the New World

Meeting between Motecuhzoma ll and Cortez with Dona Marina.

In the western hemisphere the history of disease developed differently: many highly developed civilisations existed which around 1500 had a level of development comparable to Europe, with large cities and much traffic; but without diseases. Most probably this is because their domesticated animals were not descended from species living in large herds with their own parasites. (They had only lamas, alpacas, cavias and dogs as domesticated animals).

The extent of deaths of the Indians through disease has for long been underestimated because the population had been estimated much smaller than it was: there must have been about one hundred million people in the Americas at the time of Columbus, of which 25 to 30 million lived in Mexico. Thirty years later there were still three million in Mexico and in 1620 only 1,6 million.

The journal of a Spanish expedition which sailed up the Amazon in 1542 did not mention diseases at all and described the people living along the river as numerous, strong and healthy. People who made the same trip in the 19th century (or in our times) couldn’t do this without ant-malarial medicines and met few native peoples. Malaria and yellow fever were imported from Africa.

After the European diseases had decimated the original population, black slaves were imported and they brought African diseases with them, causing more Indians to die.

The European expansion

Indians and fauna of South America, as Europeans found these in the sixteenth century. From: Wahrhafftige Historien einer wunderbaren Schiffart, 1602.