The overwhelming positive effect of the Colombian Exchange is/was the beneficial plants and animals which were introduced both to the Americas and to Europe. Cattle, pigs, sheep, chickens and horses were introduced to the Americas as well as wheat, citrus fruits, and other vegetables. Turkeys were introduced to Europe along with potatoes, tomatoes, squash and pumpkins. Both sides benefited from improved diet, especially Europe which experienced a population explosion.
The negative effect consists of the harmful elements introduced to each, primarily into the Americas. Diseases such as measles, mumps, and especially smallpox decimated native populations. On a less disastrous but still unpleasant note, the cockroach was inadvertantly introduced to the Americas by Spanish conquistadores. In turn, the Indians who were captured by Columbus and carried as hostages back to Europe introduced syphilis to that continent and an epidemic resulted.