OS X is already engineered with all the protection it requires to protect itself from viruses and malware. The question you encounter is better stated "do I need anti-virus software from those who claim to know how to protect OS X better than the company that developed and supports it" and tends to answer itself.
Installing third party anti-virus junk cannot protect your Mac from all potential threats, introduces risks of its own, complicates troubleshooting, and accounts for a significant number of problems reported on this site.
The safest way to use the Mac as designed is to use common sense - don't install something without knowing what it is supposed to do and how to get rid of it. Don't supply your user name and password unless you know the reason for doing so. Don't click on unknown links in random emails. Don't download pirated software. Beware of phishing attempts. Avoid any product that claims to "clean up", "speed up", "optimize" etc because they will all do exactly the opposite. Keep OS X up to date with software updates from Apple. That is all.
You may find this User Tip on Viruses, Trojan Detection and Removal, as well as general Internet Security and Privacy, useful: The User Tip seeks to offer guidance on the main security threats and how to avoid them.
More useful information can also be found here: