Explorers & natives - who infects whom?

It seems there are many cases throughout history where a group of explorers makes first contact with some group of natives, and the natives get infected by diseases with which they had no previous contact or immunity. Europeans came to America and the native Americans got smallpox and other such diseases… and many other examples throughout history. I remember reading about how the Spanish inadvertently infected the Mayans and that epidemic wiped out a vast majority of them in a pretty short time.

It seems the visitors/explorers always infect the natives, but not vice-versa. Why is that? I never hear about the new arrivals getting infected from the natives.

Maybe i’m wrong about this and have not read the right things, or maybe it’s a case of selective reporting. But it seems two groups of people who had been separated by vast distances would each have different diseases they are both immune and susceptible to. For example, were there no American diseases that Europeans were highly susceptible to? Why is it that the visitors always seem to infect the natives but not the other way around? Seems to be a strange lack of parity…

Biodiversity. Look at a map of the world. The Old World consists of Africa, Asia, and Europe. The colonists were from the Old World, meaning they were exposed to a wider variety of pathogens and the population had already developed some amount of stabilization (if not immunity) to these diseases.

In comparison, native populations in the Americas were relatively isolated. The American continent also had fewer domesticated animals compared to the Old World. Many human diseases initially arise in domesticated animals (e.g., the flu virus from chicken), so you can see how human populations living in the Old World would be exposed to more pathogens than those living in the New World.

It goes both ways. Lot’s of explorers died from malaria, for example, killed a lot of explorers. Syphilis is thought to have come to Europe form explorers returning from North America. The black death came back with explorers/traders and wiped out a large portion of Europe’s population.

I don’t think Europeans brought any epidemics to Africa, but those colonizers died in droves from African diseases.

Well malaria is from mosquitoes… i was referring to diseases spread from one group of people to another (vector = humans). But europeans definitely had weakness to all sorts of tropical diseases.

I was thinking that the old worlders doing the exploring came from “dirtier” societies, but i don’t think that’s a very comprehensive answer. I could be wrong though.

That’s a possibility, but has never been confirmed and alternative theories are considered quite viable. The only real evidence is that records of Syphilis don’t appear in European history beforehand, but that’s hardly conclusive as we know it mutated historically. For example, descriptions of what is believed to be Syphilis among early explorers in America sometimes note extreme manifestations.

Many infectious diseases also didn’t have a great deal of opportunity to spread around the tribal societies inhabiting much of North and South America. There’s only a limited range of possible infection vectors, virulence, and etc that can survive. However, the city-based societies of Central America had more extensive trade (although the links extended very far north and south) and considerable population density.

Traders, not explorers as far as we know. The Black Death in one form or another infested the entire old world. It’s absolutely true that infectious diseases are not the province of Europe: they were shared around the entire planet except the Americas.

Did Westerners cause any outbreaks when they first arrived in Japan (which, before then, was for the most part a closed society)? I’ve never heard anything about it one way or the other… I’m assuming westerners didn’t catch any japanese-borne diseases.

I’m not sure why you would want to make that distinction. Malaria is spread between humans, just via a mosquito intermediate host. If you don’t have humans infected with malaria present, then there can be no transmission to other humans. There is no long-term reservoir in mosquitoes or other animals. Both malaria and yellow fever were introduced to the Americas from the Old World, most likely coming from Africa via slaves. Europeans in West Africa suffered great mortality from both diseases because they had less resistance than the native populations that had been exposed to them longer.

I think it probably went both ways.

Well, “natives” is a pretty vague term and covers a pretty broad range of encounters.

That said, the reasons behind the typically one-way spread of diseases are simple and easy to understand. The book Guns, Germs, and Steel, which I constantly recommend to people, lays them out especially clearly.

We are talking about epidemic diseases specifically. These diseases are caused by germs that infect an individual, making that person sick and possibly killing him or her. Typically the victims who recover are then immune to that disease.

You may already see the problem, evolutionarily speaking. Why doesn’t the disease die out, if it spreads around making survivors immune?

It DOES die out, unless the victim population is both large and dense. Large and dense populations allow an epidemic disease to remain “alive” because as it spreads through the herd making every survivor immune, it takes so long that new, non-immune individuals can be born, to become new victims.

My use of the term “herd” is intentional. Overwhelmingly, epidemic diseases evolve in herd animals, because only herd animals have large, dense populations. In particular, humans living as hunter-gatherers do NOT have large, dense populations. Only after the advent of settled farming did such conditions arise. Coincidentally, farming in Eurasia included domestication of herd animals, exposing humans to sources of epidemic disease that could mutate and leap across the species barrier to become human diseases.

Why Eurasia specifically is addressed in G,G&S; basically, the distribution of domesticable large animals happened to be heavily Eurasian. There are surprisingly few species with appropriate characteristics to be domesticated – Jared Diamond counted something like 14. Also, Eurasia being wide helped (seriously – agriculture is constrained by climate and difficult to spread north/south, but not limited by east/west migration).

Thus people from Europe and Asia had a long history of exposure to epidemic diseases. They both built up genetic resistance to these diseases and carried them with them to their encounters with small, scattered populations in the north/south oriented continents (the Americas, and to a significantly lesser extent, Africa) and the islands.

Forgot to add – essentially all human epidemic diseases have been traced to origins in herd/flock animals. Influenza from poultry and pigs; smallpox from cowpox; measles from, I believe, rinderpest (cows); chicken pox; plague from rats; and so on and so on.

Japan had been part of the global trading network and had been exposed to all the diseases of Eurasia. Same thing with Africa. You only had the major outbreak effect when you encountered a population that had been completely isolated like Australia or the Americas.

That sentence makes it sound as if “infectious diseases” as a group didn’t exist in the Americas before the arrival of the Genoese and his merry boys: there were infectious diseases there (and in the Pacific Islands, and in Australia…), but the explorers brought a bunch of new ones to which the native populations were particularly vulnerable (the foreparents of the explorers having been culled generations before).

You (and Sailboat) are correct: I should have said epidemic diseases. My apologies.

One additional point to the many good reasons above.

Many of the diseases that killed Europeans when they went exploring and colonizing were what are now called tropical diseases. They require vectors that don’t flourish in temperate climates. Malaria was certainly known in Europe, but it was mainly a disease of the Mediterranean countries. The Anopheles mosquito likes warmer climates. So does the yellow fever mosquito. And the tse tse fly.

Plague outbreaks have historically occurred mainly in warmer climates; the one that hit Northern Europe as the Black Death spread from people fleeing southern Europe. Even smallpox started in warm areas, probably in Egypt.

The one-way nature of illness has multiple explanations, but together they’ve very powerful. The degree of immunity the Europeans had built up by being exposed to their own and trade-spread diseases for centuries, along with the inability of disease vectors to flourish in cooler climates, made them universal donors (analogous to those with type-O blood) and the Native Americans universal recipients (analogous to those with type-AB blood).