Often credited with having "discovered" North America, Christopher Columbus had positive and negative effects on the world. His voyages helped establish new trade routes and bring new goods to England. On the down side, his arrival brought infectious diseases that wiped out Native American populations, and the colonization that followed destroyed thousands of years of Native American culture.
The damage that Columbus' voyages caused to Native American populations came in several forms. To find riches in the "New World," Columbus' men committed acts of violence against the Native Americans, driving them off their land and taking their resources. Some Native Americans were forced into slavery. Thousands of others died from smallpox and other infections brought over by the European explorers. Since they had no prior exposure to the infectious agents that caused these diseases, Native Americans had no immunity and were thus highly susceptible.
In spite of the damage caused by Columbus' voyages, his work also had some positive impacts on the world. He pioneered new sailing and navigating techniques, which allowed Europeans to travel further than ever before and more safely. He brought plants and animals from the Americas back to Europe, providing his people with a greater array of resources.