There are many reasons why diseases spread so rapidly. It is simply because the diseases were present. How is the more interesting question. The Native Americans had built up immunities to diseases and viruses that infected people in the Americas. They had never been exposed to anything from Britain, Spain, France, etc. etc. The body is unable to protect itself against new bacteria in the system. Especially if the bacteria is introduced that quickly. For example, (something you can relate to) say you get the common cold every two weeks. Slowly, your body starts to fight off symptoms of the cold. First the sneezing, then the runny nose, next the watery eyes, and finally the coughing. You still get the cold every two weeks but is not nearly as bad. But, let's say you get the stomach flu. Your immunity to the common cold will do nothing to help you with the stomach flu you now have.
