Although the Columbian exchange is said to have had some positive outcomes, the negative outcomes far outweigh and outlast many of the said positive outcomes. The appearance of Christopher Columbus in the Americas facilitated the spreading of plants, pathogens, and diseases from the Old World to the New World.
The major, most prominent negative outcomes were the proliferation of slavery and the spread of transmittable infections and diseases. The Europeans carried huge loads of transmittable illnesses to the Americans, causing an estimated 95% of natives to succumb to the illnesses and die. While the Europeans had some sort of immunity against the diseases such as smallpox and malaria which they brought, the natives, being previously unexposed to these diseases, were unprotected against these deadly diseases.
Also, natives who refused to accept the faith of the Europeans were publicly persecuted until they either yielded or died -after enslaving them, of course.