There's always room for discussion, but perhaps the section Impact of indigenous diseases on the Europeans from our FAQ will answer your inquiry.

It was really a numbers game. In the Old World there were more domestic animals on more acres, able to spread to more land.

Europeans died from New World diseases, but there were fewer of these. The main New World-origin offender seems to have been syphilis, caused by Treponema pallidum. T. Pallidum was already in the Old World, but the strain there caused yaws, a disease with a different presentation that was rare in Europe but commoner father south. The New World strain of Treponema pallidum was far more aggressive, and in decades after Columbus it spread to Europe and killed perhaps ten millions of people.

There were fewer infectious diseases of humans in the New World for a couple of reasons. The American continents are arranged from north to south. Ecological reasons (rainfall, temperature) kept microbes from spreading over as wide areas as in the Old World, where ecosystems were ranged east/west over much longer distances, longer by thousands of miles. This allowed a greater mutation potential of microbes. Old World geography made for ecosystems that were simply bigger, because Europe, Asia, and Africa had more land in a given biome. As an example, the Eurasian steppe zone stretches for about five thousand miles from Hungary to China. Nothing in the New World comes close to that.

The New World indigenous peoples had no domestic animals that existed in huge herds. Old World domestic animals with huge herds likewise afforded microbes a wider scope and bigger environment. The microbes in these Old World animals simply had a better chance of developing mutations able to infect humans. They also had a better chance of infecting people over a wider area, and all these factors presented American indigenous peoples with a wide population of microbes and viruses that they had no immunity to.

Keep in mind that when syphilis spread to the Old World it caused a huge death toll, but syphilis was just one disease entering a virgin population, whereas the Old World had many diseases to contribute in the opposite direction.