Hey guys I was wondering do I need an anti virus software on my mac? I don't think I need it but as a first time mac user I wanted your opinion.

Question

Just wanted to get your opinion.

Answer 1

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 deliberately 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:
It doesn't apply to software that comes packaged as an installer. Treat all third-party installers with caution.
It can be disabled or overridden by the user.
It can be bypassed by some third-party networking software, such as BitTorrent clients and Java applets (see below.)
It only applies to applications downloaded from the network. Software installed from a CD or other media is not checked.
For more information about Gatekeeper, see this Apple Support article. 
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?
Any website that prompts you to install a “codec,” “plug-in,” or “certificate” that comes from that same site, or an unknown one, is untrustworthy.
A web operator who tells you that you have a “virus,” or that anything else is wrong with your computer, or that you have won a prize in a contest you never entered, is trying to commit a crime with you as the victim. [Some reputable websites did legitimately warn users who were infected with the "DNSChanger" malware. That exception to this rule no longer applies.]
“Cracked” copies of commercial software downloaded from a bittorrent are likely to be infected.
Software with a corporate brand, such as Adobe Flash Player, must be downloaded directly from the developer’s website. No intermediary is acceptable.
Java on the network (not to be confused with JavaScript, to which it's not related) is always a potential weak spot in the security of any operating system. If a Java web plugin is not installed, don't install it unless you really need it. If it is installed, you should disable it (not JavaScript) in your web browsers. Few websites have Java content nowadays, so you won’t be missing much. This setting is mandatory in OS X 10.5.8 or earlier, because Java in those obsolete versions has known security flaws that make it unsafe to use on the Internet. The flaws will never be fixed. Regardless of version, experience has shown that Java can never be fully trusted, even if no vulnerabilities are publicly known at the moment.
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.