Operating systems such as Microsoft Windows and Apple Mac OS X are complex software products made up of thousands of individual program files. Although Microsoft and Apple have rigorous quality control procedures, it's a fact of life that, from time to time, defects, or "bugs," will be discovered. Fortunately, as these defects are discovered, both Microsoft and Apple provide fixes, normally called security patches, and make them freely available via automatic update mechanisms or via their support web sites. It's important to keep your computer up-to-date with these security patches; if you don't, it's much more likely that your computer could be infected with malicious software (viruses, worms, etc.) or accessed by a hacker.