The main problem with thinking about this is that the question as phrased omits the single most important issue—namely, for whom? In other words, effects are not good or bad in absolute terms; they have good and bad effects on specific individuals and groups. Overall, one could say that Europeans vastly benefited from the Columbian exchange, and Native Americans suffered.
For Europeans, there were many benefits. The Europeans acquired many different crops, including tomatoes, maize, and potatoes, which became staple parts of their diet. On a more ambiguous level, tobacco was an American plant that became the basis of vast wealth, but, as we know know, its use has many harmful effects. In terms of diseases, syphilis was probably introduced to Europe from the New World. Although it is a harmful disease, it does not kill people quickly or have massive effects on populations.
The effects of the Columbian exchange on Indigenous populations, though, were horrific. Epidemic diseases such as...
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