Why were microbes the most influential aspect of the Columbian exchange?

Smallpox and Other Diseases-The most influential aspect of the Columbian exchange, however, was the exchange of microbes, viruses and germs. Because Europeans had a long history of exposure to many diseases, and because many Europeans lived in somewhat densely populated towns and cities, they had acquired some immunity to disease. Smallpox, measles, and many other diseases were carried across the Atlantic beginning with Columbus's voyage. Indians, in contrast, had no exposure to many European diseases, and many Indians lived in sparsely settled areas, so that viruses did not spread widely or rapidly.

When Indians encountered viruses for which they had no immunity, these diseases spread very rapidly and produced disastrous results. As much as three-quarters of Indians, and in some areas upwards of ninety percent of them, were killed by smallpox and other diseases. Smallpox left the skin covered with sores and usual proved fatal. The sudden drop in Indian populations proved calamitous for many Indians, wiping out villages, leaving Indians too ill to care for one another, and even undercutting Indians' faith in their own culture. Because the existence of microbes was unknown until centuries later, Indians could only wonder why so many of their people fell sick and died while most white men did not. Because Indians believed that the world was full of spirits, they despaired, wondering why the spirits seemed to favor Europeans. Both physically and spiritually, epidemics ravaged Indian societies.