Why didn't it work symmetrically? Shouldn't the diseases going the other way have wiped out Europe? The only major disease I can recall coming from the Americas was syphilis.
In simple terms, the Old World had a larger human population pool, with a longer history of settlement, and more domesticated animals (i.e. vectors for disease).
Or in simplest terms, Old World germs were way nastier. (Contrast: African settlement.)
See Guns, Germs, and Steel by Jared Diamond (which some disagree with, but an interesting book nonetheless). He says that the key factor was the large number of animal species in the old world that could be domesticated. So Europeans all had constant contact with animals and their diseases, which led to (1) more diseases, and (2) greater resistance (among those that survived). While in the Americas, with their relative lack of such species, the people had both fewer diseases (so they were less of a threat to the Europeans) and less resistance (so the Europeans were more of a threat to them).
Eurasians/Africans evolved an immune system capable of fighting infectious diseases because for thousands of years they lived in close quarters with domesticated animals and other people.
American Indians had few domesticated animals (only Llamas and Alpacas, I think), minimal over overcrowding and hence had few infectious disease. This caused them to be tall and healthy by contemporary Eurasian standards, but also made them prone to illness.
For much the same reason, I got food poisoning twice in two weeks in Egypt but my (Asian) Indian girlfriend came away unscathed.
