There must have been some, right?
As someone with some biology background, its possible that there were disease that would have decimated the European population as Small pox did to the Native American population. The difference between the two is that for the disease to spread, it needs a viable host that would survive the trip between Americas and Europe. Chances are, any disease that would have affected the Europeans as small pox did to Native Americans probably never made it past the voyage and thus it isnt noted in history. Syphilis is an American disease that spread to Europe though i believe although this is disputed.
hi! you'll find some additional info in the FAQ (link on sidebar):
Syphilis, an American disease, ravaged Old World, it quickly spread throughout Europe and Asia after its arrival in Naples in 1594. City officials in Lyons expelled people suffering from the disease to avoid the spread of contagion. Officials in Geneva forbid entrance to the city by syphilitics, and suffers were quarantined in Edinburgh. Syphilis spread rapidly from Naples to France, Germany, and Switzerland within one year. It appeared in Holland, England and Greece in 1496; the Middle East and India in 1498; Hungary and Russia in 1499; China in 1505; and Japan by 1512.
Syphilis was a "stronger" disease when first introduced to Europe - causing more visible and devastating damage than the modern disease - rotting off noses, genitals, and causing open sores on arms and legs. By the 1550s physicians started to note that syphilis had become less virulent, weaker, and more manageable over time.
Native Americans lived a lot cleaner than Europe in that time period. While the Europeans spent hundreds of years living with animals and their filth, they became accustomed to their diseases. Americans didn't have as many animals that they lived with, and so had fewer diseases. When the Europeans came over, they brought their multitude of diseases and sturdier immune systems. The Americans were sitting ducks.