The Americas were isolated for about 10.000 year from the rest of the world after the Bering bridge was flooded and people could not cross over from Asia any more.

Europe, Asia and Africa on the other hand are all geographically interconnected and had trade throughout much of those 10.000 years. When a new virus cropped up in one place, it inevitably travelled along those trade routes and would spread throughout the three continents. The Black Death pandemic in Europe was very likely caused by trade through the Silk Road and originated in Asia.

People in Europe, Asia and to a lesser extent Africa would regularly be exposed to new pathogens from other places of the world, while the Americas were completely cut off from those diseases and had no chance to obtain immunity.

When the first Europeans arrived they brought diseases that had circulated in Asia and Africa as well for centuries and large swathes of the populations there would have had some kind of immunity to. The Native Americans on the other hand had none. Had someone from Asia first stepped foot in he Americas at this time, they would have brought the same or similarly deadly diseases.

The one really nasty disease Europeans seem to have brought back from the Americas is syphilis. It's first mentioned a few years after Columbus and today thought to have likely originated from the Americas.

If you'd like to know more you can read up on the Columbian Exchange.