Sophos should be avoided, as it could actually increase your Mac's vulnerability, as described,. Currently there are zero viruses affecting Mac OS X in public circulation, but there are other kinds of malware existing, that can infect your Mac.

But as long as you don't install software from unknown and untrusted sources, you are safe, as malware needs administrative permissions to run successfully, which means, you need to install the malware yourself, it can't install itself (one of the reasons, why a Mac OS X virus hasn't appeared yet). To learn more about malware in Mac OS X and what steps can be taken to protect yourself, read the following F.A.Q. ​.

Make sure your built-in Mac firewall is enabled in System Preferences Security Firewall. Uncheck 'Open 'safe' files after downloading' in Safari Preferences General. Uncheck 'Enable Java' in Safari Preferences Security. This will protect you from malware that exploits Java in your browser, including the recent. Leave this unchecked until you visit a trusted site that requires Java, then re-enable only for your visit to that site. (This is not to be confused with JavaScript, which you should leave enabled.).

Change your DNS servers to OpenDNS servers by reading. Be careful to only install software from trusted, reputable sites. Never install pirated software. If you're not sure about an app, ask in this forum before installing. Never let someone else have access to install anything on your Mac. Don't open files that you receive from unknown or untrusted sources.

For added security, make sure all network, email, financial and other important passwords are long and complex, including upper and lower case letters, numbers and special characters. Always keep your Mac and application software updated. Use Software Update for your Mac software. For other software, it's safer to get updates from the developer's site or from the menu item 'Check for updates', rather than installing from any notification window that pops up while you're surfing the web. That's all you need to do to keep your Mac completely free of any virus, trojan, spyware, keylogger, or other malware.

You don't need any 3rd party software to keep your Mac secure. If you still want to run antivirus for some reason, (which is free) is one of the best choices, since it isn't a resource hog, detects both Mac and Windows malware and doesn't run with elevated privileges. You can run scans when you choose, rather than leaving it running all the time, slowing your system. ClamXav has a Sentry feature which, if enabled, will use significant system resources to constantly scan. Disable the Sentry feature.

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You don't need it. Also, when you first install ClamXav, as with many antivirus apps, it may perform an initial full system scan, which will consume resources.

Once the initial scan is complete, periodic on-demand scans will have much lower demands on resources.