OS X is resistant to viruses by design, but there is no such thing as being immune to malware. This has become an urban myth. Vulnerabilities can crop up in OS, third party programs, or in clever social engineering malware. Keeping the system updated is the key defense.
Macs already come with "basic" anti-malware protection built-in
XProtect(File Quarantine) since Snow Leopard. Apple also provides guides for
hardening security on older OS X versions. Personally, I prefer and have installed a full AV package on all my Macs, several which are available for free. I've had no issues or noticed any slowdowns on my MacBook with them installed.