Re: Why didn't early explorers take new diseases home with them?

Hello, and welcome to the MadSci network. Your question is very well written, 
and indicates that you are a very logical and intelligent thinker. Many people 
have wondered about the same question you asked, for many years.

There is a possibility that the diseases brought to the "New World" by European 
explorers were diseases that developed within crowded population groups in 
Europe. The "New World" inhabitants did not tend to live in overcrowded cities, 
and thus did not have those kinds of diseases.

If an Explorer in the "New World" suffered from one of the diseases that were in 
the "New World" area he was exploring, it is more likely to be a very deadly 
disease, or a bite from a poisonous animal, and the Explorer usually died from 
that disease or poison before he could return back to Europe. 

However, in the crowded cities of Europe, there were diseases developed that did 
not kill people rapidly. For example, sexually-transmitted diseases can last 
within a human body for many years. Other diseases that developed in the 
European populations also had slower progression within the body. Therefore, it 
is likely that some of these diseases were carried by the Europeans on their 
travels.

There is also the fact that some of the Europeans who tried to defeat the 
indigenous populations, including some of the Native American tribes in North 
America, actually deliberately contaminated the indigenous populations. In some 
cases, a vaccine or other preventative measure protected the European settlers, 
but the indigenous people had no protection at all, if the Europeans 
deliberately gave them products or items that had been infected with a specific 
European disease. Some of this information has been discovered in recent years 
about this matter, which was not known in previous years. Biological warfare has 
been known to the Europeans for a long time.

Dan Berger adds:
The first to use biological warfare were the Tatars, a central Asian people,
who used to catapult diseased corpses into cities they were besieging. It is
known that some Great Plains tribes were infected with smallpox by gifts of
blankets, but it is not clear to me whether the infection was deliberate.

In fact, deliberate infection was not usually necessary--the Massachusetts
Bay colonists ("Pilgrims") were able to settle in so quickly because some
disease, probably smallpox caught from a whaling crew, had wiped out several 
local villages. The local king was happy to get people who would occupy the 
land, so that rival tribes couldn't take it over.

There were no vaccines during most of the European exploration of the Americas.
It's just that, as Paulette notes, most New World peoples did not live in
cities and so didn't have the sort of constant immune challenges that 
city-dwellers have.

But that doesn't explain why the Aztecs, who lived in large cities--some of them 
larger than any in Europe at the time--died of measles by the tens of thousands.

Diseases such as measles, which the Europeans had had as children,
were unknown in the New World--but there weren't similar "childhood diseases"
in the New World for the Europeans to get. If there were, as Paulette notes,
the explorers didn't live long enough to bring the disease home (with the 
possible exception of syphilis).

This is, I think, the key: If American explorers had come to Europe, it's 
entirely possible that the same sort of mass epidemics would have been seen. But 
explorers who contract serious illnesses for which their bodies are unprepared, 
usually don't make it home, so there isn't the opportunity for reciprocal 
epidemics.