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Professor Bernard Ortiz de Montellano

Question for November 2010

Would there have been more diseases among the English than among the Aztecs at that time? Asked by The Blue Coat School. Chosen and answered by Professor Bernard Ortiz de Montellano.

English and Aztec family life c.1500
English and Aztec family life c.1500 (Click on image to enlarge)

The short answer to the question is that the English in the 1500’s had more diseases than the Aztecs. However, to do justice to the question even in a simple way, we need to briefly discuss the different kinds of diseases and the history of infectious diseases.

Diseases can be roughly classified as chronic, parasitic or infectious. Chronic diseases such as cancer, heart attack, diabetes, and stroke take time to develop and principally affect older people. In AD 1500, most Aztecs or Englishmen did not live to ripe old ages and therefore these diseases were not the main causes of death, as they are today. Even so, because of their diet and level of activity, the Aztecs probably had a somewhat smaller incidence of these diseases than the English.

Roundworms infest the small intestine
Roundworms infest the small intestine

Parasitic infections by such organisms as roundworms, hookworms, whipworms, or flatworms infected both Aztecs and the English. Some, like the beef tapeworm were not present in the New World because they require the presence of cattle and/or pigs and the Aztecs did not have them. On the other hand, the lung fluke and the roundworm were present in the New World but not in England. Apparently, parasitic infections do not distinguish between England and the Aztecs.

Neutrophil (part of our body’s defense system) attacking a pathogen
Neutrophil (part of our body’s defense system) attacking a pathogen

The crucial difference, thus, is the incidence of infectious disease, i.e., a disease that can be passed from person to person. An important concept to keep in mind is that of immunity. Until the discovery and wide use of antibiotics in the Twentieth Century, the main defensive mechanism for people was the immune system. When a germ infects us, our body mobilizes a complex array of defenses to destroy the infectious organism. If our immune system wins, we don’t get sick, however, if the infectious organism is able to withstand the attack of the immune system, we get sick with that disease. Because the immune system has to build specific defenses for each particular germ, it makes a big difference if this specific defense can be built quickly. One factor is the ability of the immune system to identify which germ is invading the body. For example, if someone is infected with smallpox and manages to survive, his immune system will recognize and act swiftly to any subsequent infection by smallpox. This is what vaccination is supposed to do. If we can prime the immune system by presenting a mild or innocuous infection, it will be able to mount a fast response to a subsequent real infection by this particular infectious agent.

Agriculture: ‘The Harvesters’ by Pieter Bruegel
Agriculture: ‘The Harvesters’ by Pieter Bruegel (Click on image to enlarge)

For most of their existence (i.e. approximately from 190,000 BC to 10,000BC) modern humans lived in small hunter-gathering bands (50-100 people). If a virulent germ infected one of these bands, the result was that most members of the band would die and that those that survived would be immune to further infection by that organism. The good news for the germ was that it successfully attacked the humans, but the bad news for the germ was that there was no one left to infect and it would die out. This meant that, as long as humans lived as hunter-gatherers, infectious diseases were not a major problem, because even if new infectious diseases arose, they would die out. The domestication of plants and animals, and the beginning of agriculture changed this dynamic.

Nuremberg in the Middle Ages
Nuremberg in the Middle Ages (Click on image to enlarge)

First, the surplus food produced made it possible to have larger and larger settlements that eventually culminated in large cities. And, second, the fact that people remained permanently in one place instead of moving from one place to another, created a problems of hygiene such as the disposal of garbage and human waste, the provision of uncontaminated water and an increase in pollution. In the old days, cities were very unhealthy places.

The measles virus
The measles virus (Click on image to enlarge)

More relevant to our discussion is the fact that larger populations made “herd” infectious diseases possible. Let’s compare our previous example of an attack on a small band by a virulent germ with an attack of the same germ in a city with a population of 100,000. In the beginning, just as in the previous example, most of the population would get sick rapidly and many would die and be left with some immune survivors. This is what we call an epidemic. However, due to the size of the population the germ would not go extinct because new non-resistant babies would be born that could be infected. Another consequence is that the virulence of the infectious agent decreases over time. When an infectious disease exists at a low level and with reduced virulence, we call it an endemic infectious disease.

The Black Death
The Black Death (Click on image to enlarge)

In the Old World, measles, for example was an endemic disease. Children got a mild case of measles and then had immunity for the rest of their lives. It takes a population of about 300,000 people in contact with each other to keep measles alive as an endemic disease. In the Old World cities with large populations had been in contact with each other through trade and warfare for thousands of years. We know that they suffered periodic epidemics including such terrible ones as the “Black Death” plague in the 14th Century. Certain herd diseases, like measles and smallpox, had become endemic in Europe, including England.

Smallpox hits the Aztecs - Florentine Codex Book 12
Smallpox hits the Aztecs - Florentine Codex Book 12 (Click on image to enlarge)

However, “virgin populations”, those who have not been exposed previously, can have a very high mortality when exposed to diseases that are endemic elsewhere. The best evidence that the Aztecs had fewer diseases than the Europeans, such as the English, is the fact that, in the first century after the conquest of Mexico, they had a series of terrible epidemics with very high mortalities from diseases such a measles, smallpox, and typhus which were endemic in England. That is, the Aztecs were a “virgin population” to a number of European infectious diseases. An immediate effect was that a smallpox epidemic during the Spanish siege of Tenochtitlan was a big factor in the ultimate conquest of Mexico.

Plague victims in Europe
Plague victims in Europe (Click on image to enlarge)

We can now compare the principal infectious diseases afflicting the Aztec and the English populations:-

Aztec and English diseases
• Dysentery
• Viral influenza
• Pneumonia
• Tuberculosis (disputed in New World)
English only diseases
• Measles
• Smallpox
• Whooping cough
• Scarlet fever
• Typhus
• Leprosy
• Diphtheria
• Plague
• Mumps
Aztec only diseases
• Syphilis (disputed)

Milking cows in ancient Egypt
Milking cows in ancient Egypt (Click on image to enlarge)

There are a couple of reasons for the difference in the number of infectious diseases afflicting the Aztec compared to those affecting England and the Old World. The most significant factor was the smaller probability of new human infectious diseases developing in the New World relative to the Old World. Infectious diseases commonly begin as diseases of animals that adapt and jump to humans. For example, measles came from rinderpest in cattle, mumps began in pigs, and influenza came from ducks and pigs. Usually this transmission requires close continuous contact between animals and people. The domestication of animals some 10,000 years ago began this process.

The llama
The llama (Click on image to enlarge)

Herders live with their herds, some of these animals become pets and often domesticated animals live in the same house as their owners. There is a huge difference in the number of domesticated animals in the Old and the New Worlds. Of the 14 major domestic mammals, 13 were domesticated in the Old World (including cows, sheep, goats, pigs and horses) and only one, the llama, in the New World. The Aztecs only domesticated dogs and turkeys (turkeys are not very cuddly). This difference in the number of domesticated animals is a clear explanation for the few infectious diseases afflicting the Aztecs.

The European city of Haarlem in the Middle Ages
The European city of Haarlem in the Middle Ages (Click on image to enlarge)

Second, even if infectious diseases had developed in the New World, cities large enough to support herd diseases had not been present as long in the Americas as in the Old World. The large population centers in the New World - Mesoamerica, the Andes, and Mississippi Valley did not trade fast enough or continuously enough to form a herd disease sustaining population.

To sum up: Of the different kinds of diseases neither chronic nor parasitic diseases establish a clear difference between England and the Aztecs. However, the Aztec had far fewer infectious diseases than Europeans like the English. Sadly, this led to a huge population decline among the Aztecs (and all the other inhabitants of the New world) due to epidemics of European infectious diseases in the first centuries after the discovery of America.

Picture sources:-
• English family life: from
• Aztec family life: illustration by Alberto Beltrán
• Roundworms: from
• All other images from Wikipedia, except -
• Image from the Florentine Codex (original in the Biblioteca Medicea Laurenziana, Florence) scanned from our own copy of the Club Internacional del Libro 3-volume facsimile edition, Madrid, 1994

Professor Bernard Ortiz de Montellano has answered 3 questions altogether:

How much energy can you get from one cocoa bean?

Would there have been more diseases among the English than among the Aztecs at that time?

How did the Aztecs know that this (fifth) world would end in an earthquake?

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