do you need anti virus on a Mac?
Unfortunately, your question has drawn the attention of someone who habitually posts misinformation on this site.
This comment applies to malicious software ("malware") that's installed unwittingly by the victim of a network attack. It does not apply to software, such as keystroke loggers, that may be installed by an attacker who has hands-on access to the victim's computer. That threat is in a different category, and there's no easy way to defend against it. If you have reason to suspect that you're the target of such an attack, you need expert help.
All versions of OS X since 10.6.7 have been able to detect known Mac malware in downloaded files. The recognition database is automatically updated once a day; however, you shouldn't rely on it, because the attackers are always at least a day ahead of the defenders. In most cases, there’s no benefit from any other automated protection against malware.
Starting with OS X 10.7.5, there is another layer of built-in malware protection, designated "Gatekeeper" by Apple. By default, applications that are downloaded from the network will only run if they're digitally signed by a developer with a certificate issued by Apple. Applications certified in this way haven't actually been tested by Apple (unless they come from the Mac App Store), but you can be sure that they haven't been modified by anyone other than the developer, and his identity is known, so he could be held responsible if he knowingly released malware. For most practical purposes, applications recognized by Gatekeeper as signed can be considered safe. Note, however, that there are some caveats concerning Gatekeeper:
Notwithstanding the above, the most effective defense against malware attacks is your own intelligence. All known malware on the Internet that affects a fully-updated installation of OS X 10.6 or later takes the form of so-called "trojan horses," which can only have an effect if the victim is duped into running them. If you're smarter than the malware attacker thinks you are, you won't be duped. That means, primarily, that you never install software from an untrustworthy source. How do you know a source is untrustworthy?
Follow these guidelines, and you’ll be as safe from malware as you can reasonably be.
Never install any commercial "anti-virus" products for the Mac, as they all do more harm than good. If you need to be able to detect Windows malware in your files, use the free software ClamXav — nothing else.