In the book Guns Germs and Steel, Explain why the exchange of diseases from the old world (Europe) and the new world (america) was so one-sided (America got killed off, not europe). In doing so, make sure to explain exactly what is a "Social" animal.
When one country infects another, it is usually because the people in the new country do not have immunity to the illness. The illnesses don't go the other way because there are no diseases in the new country to infect the other. America did not have any diseases to send to Europe.
The book argues that one of the principal reasons for this phenomenon was the way in which the Old World had more developed cities and more domestic animals, which brought humans in closer contact with each other and with a larger range of bugs and germs. Of course, let us remember that the Old World itself had to face numerous plagues and illnesses, as the Black Death should help us remember. However, the lack of domestic animals in the new world meant they were not exposed to the same degree, making them ripe for small pox and other diseases.
There is a strong argument by a number of historians that the exchange was not entirely one-sided. Although Europeans carried a resistance to small pox, measles, etc. to which native Americans had never been exposed; it seems that the native Americans also carried a resistance to syphilis to which the Europeans had not been exposed. As a result of sexual contact with the Indians in the Americas as well as with Indians carried back to Europe, a virtual epidemic of syphilis erupted in Europe.
Incidentally, "survival of the fittest" was not a term that Charles Darwin ever used. It was the term coined by Herbert Spencer to support Social Darwinism, his position that Europeans were naturally and genetically superior to others.
Another thing to consider is the paradigm of social determinism: Only those who can adapt to change and to the many different circumstances that occur to individuals will be likely to survive just about anything that comes their way. In the case of the Europeans, they were already resistant to many of the strains of bacteria and virus that they carried. Being more exposed to a virus or bacteria will consequently allow you to produce more antibodies that are needed to fight them. At some points, some illnesses are dormant, or asymptomatic. Yet, in a fresh, healthy, and untouched body with little exposure to disease, the development of antibodies will also consequently manifest as a first bout of the disease, which is often the most violent manifestation of it. For example, being exposed to measles or E.Coli can be deadly for a baby whose immune system is free from antibodies. The bodies of the Native Americans were very much like babies when it came to the white men's diseases. This is why it seems one-sided. However, not all the Natives died and this is when the idea of the "survival of the fittest" comes into play.
The Old World had all sorts of infectious diseases and the New World didn't because the Old World was the one that had most of the domesticated animals and those are the animals that tend to be "social."
Social animals are ones that tend to get together in herds rather than running around alone. They're the ones that get epidemic diseases. When those animals get domesticated, they pass their diseases to people. The Old World had lots of social animals and therefore lots of domesticated animals. That's why it had epidemic diseases to pass to the New World rather than vice versa.