Answer:
The positive effect is the exchange that happened between the Old and New World, and the negative effect are the diseases that Europeans brought to America that killed millions of Natives.
Step-by-step explanation:
After Columbus discovered new continent many products that were unknown to people on one or other continent were exchanged. That was probably the most positive consequence of the Columbus discovery.
On the other side, diseases such as smallpox killed millions of Natives, which together with conquistadores led to extinction of certain tribes.