Most CT scans (often pronounced "cat scans") have low radiation risk because they are targeted at specific areas of the body. They are ordered because a health care provider has reason to believe there is a disease or other problem with that part of the body, so the benefits outweigh the risks.
Full-body CT scans deliver higher radiation levels, which are known to increase cancer risk. Healthy people should avoid this risk.
The American Cancer Society and the American College of Radiology both discourage full-body CT scanning.