NONE!!!!! There are zero (as in none, zip, they don't exist) viruses for OS X. Antivirus software for a Mac is a waste of time, resources and possibly money. If you insist on having antivirus software on a Mac then download and install ClamXav. This is the least intrusive and is free but it's a waste because there are no viruses for OS X.
No viruses that can attack OS X have so far been detected 'in the wild', i.e. in anything other than laboratory conditions.
It is possible, however, to pass on a Windows virus to another Windows user, for example through an email attachment. To prevent this all you need is the free anti-virus utility ClamXav, which you can download for Tiger and Leopard (check with them about Lion) from:
The new version for Snow Leopard is available here:
Note: ClamAV adds a new user group to your Mac. That makes it a little more difficult to remove than some apps. You’ll find an uninstaller link in ClamXav’s FAQ page online.
If you are already using ClamXav: please ensure that you have installed all recent Apple Security Updates and that your version of ClamXav is the latest available.
Do not install Norton Anti-Virus on a Mac as it can seriously damage your operating system. Norton Anti-Virus is not compatible with Apple OS X.
FAKE ANTI-VIRUS SOFTWARE and associated MALWARE
Do not be tricked by 'scareware' that tempts computer users to download fake anti-virus software that may itself be malware.
Fake anti-virus software that infect PCs with malicious code are a growing threat, according to a study by Google. Its analysis of 240m web pages over 13 months showed that fake anti-virus programs accounted for 15% of all malicious software.
Scammers trick people into downloading programs by convincing them that their PC is infected with a virus.
Once installed, the software may steal data or force people to make a payment to register the fake product.
Beware of PDF files from unknown sources. A security firm announced that by its counting, malicious Reader documents made up 80% of all exploits at the end of 2009.:
http://www.computerworld.com/s/article/9157438/in which Rogue_PDFs_account_for_80_of_all_exploits_says_researcher
TROJANS and RE-DIRECTION TO FAKE WEBSITES
The appearance of Trojans and other malware that can possibly infect a Mac seems to be growing, but is a completely different issue to viruses.
If you allow a Trojan to be installed, the user's DNS records can be modified, redirecting incoming internet traffic through the attacker's servers, where it can be hijacked and injected with malicious websites and pornographic advertisements. The trojan also installs a watchdog process that ensures the victim's (that's you!) DNS records stay modified on a minute-by-minute basis.
You can read more about how, for example, the OSX/DNSChanger Trojan works (by falsely suggesting extra codecs are required for Quicktime) here:
SecureMac has introduced a free Trojan Detection Tool for Mac OS X. It's available here:
First update the MacScan malware definitions before scanning. You can also contact their support team for any additional support - firstname.lastname@example.org [/b]
The DNSChanger Removal Tool detects and removes spyware targeting Mac OS X and allows users to check to see if the trojan has been installed on their computer; if it has, the software helps to identify and remove the offending file. After a system reboot, the users' DNS records will be repaired.
(Note that a 30 day trial version of MacScan can be downloaded free of charge from:
and this can perform a complete scan of your entire hard disk. After 30 days free trial the cost is $29.99. The full version permits you to scan selected files and folders only, as well as the entire hard disk. It will detect (and delete if you ask it to) all 'tracker cookies' that switch you to web sites you did not want to go to.)