Q: do i need a anti virus for my mac

Hello, not really, but if you like...

ClamXAV, free Virus scanner...

http://www.clamxav.com/

Free Sophos...

http://www.sophos.com/en-us/products/free-tools.aspx

Little Snitch, stops/alerts outgoing stuff...

http://www.obdev.at/products/littlesnitch/index.html

Get MacScan...

http://macscan.securemac.com/

Disable Java in your Browser settings, not JavaScript.

http://support.apple.com/kb/HT5241?viewlocale=en_US

http://support.google.com/chrome/bin/answer.py?hl=en-GB&answer=142064

http://support.mozilla.org/en-US/kb/How%20to%20turn%20off%20Java%20applets

Flashback - Detect and remove the uprising Mac OS X Trojan...

http://www.mac-and-i.net/2012/04/flashback-detect-and-remove-uprising.html

In order to avoid detection, the installer will first look for the presence of some antivirus tools and other utilities that might be present on a power user's system, which according to F-Secure include the following:

/Library/Little Snitch

/Developer/Applications/Xcode.app/Contents/MacOS/Xcode

/Applications/VirusBarrier X6.app

/Applications/iAntiVirus/iAntiVirus.app

/Applications/avast!.app

/Applications/ClamXav.app

/Applications/HTTPScoop.app

/Applications/Packet Peeper.app

If these tools are found, then the malware deletes itself in an attempt to prevent detection by those who have the means and capability to do so. Many malware programs use this behavior, as was seen in others such as the Tsunami malware bot.

http://reviews.cnet.com/8301-13727_7-57410096-263/how-to-remove-the-flashback-ma lware-from-os-x/

http://x704.net/bbs/viewtopic.php?f=8&t=5844&p=70660#p70660

The most current flashback removal instructions are F-Secure's Trojan-Downloader:OSX/Flashback.K.

https://www.securelist.com/en/blog/208193454/Flashfake_Removal_Tool_and_online_c hecking_site

More bad news...

https://www.securelist.com/en/blog/208193467/SabPub_Mac_OS_X_Backdoor_Java_Explo its_Targeted_Attacks_and_Possible_APT_link

Removal for 10.5...

http://support.apple.com/kb/DL1534

If your OS is less than OS X 10.6.7, which is when XProtect -- not foolproof either -- was introduced, then ClamX or Sophos might not be a bad idea. But don't let any A-V give you a false sense of security. 10.5 Leopard and 10.4 Tiger are no longer being supported by Apple with security patches.

Glad we could help, & a warm welcome to the forums & Macdom!

This is a pretty decent introduction to the subject.

http://www.reedcorner.net/mmg/

There are many good reasons to have an antivirus installed. 

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NO, NO, NO stay as far away from Mackeeper as possible. Aside from the very questionable ethics of their marketing practices, Mackeeper is well known for its very negative effects on Macs. Just do a search here for other threads in which this is discussed, or google Mackeeper site:discussions.apple.com

Hmm, I wonder what motivates someone to post an ad for MacKeeper on a bunch of topics here that have been inactive for more than a year or two... Couldn't be astroturfing, could it?

http://en.wikipedia.org/wiki/Astroturfing

On a scale of Maybe at 1, to Certainly at 10, I'd give it a 9.9. But as names go, "astroturfing" is really quite inventive. Hadn't seen that before. Great neologism.

I like your hat !