None. There are no OS X viruses at this time.
Helpful Links Regarding Malware Protection
An excellent link to read is Tom Reed's Mac Malware Guide.
Also, visit The XLab FAQs and read Detecting and avoiding malware and spyware.
See these Apple articles:
If you require anti-virus protection I recommend using ClamXav.
I love the Mac that I use at home, and in the past, I have never had the need for anti-virus software (that has been my belief for many years). But for the Mac I use at work. . .our company has an Enterprise Security Department that requires that all workstations and laptops have anti-virus software installed. They don't have this silly requirement just for fun; they formulated this policy to allow our company to pass security audits, and because they have a belief that threats do exist, even for Macs.
They installed Sophos Anti-Virus on my Mac to satisfy this requirement, and when it appears to be downloading updated patterns, the system crawls and is unusable for up to thirty minutes at a time. Unfortunately, I'm one of the few Mac users within our enterprise, so the solution our IT department offers to me when I complain of this poor performance is to trade in my Mac for a Windows workstation (not a solution I like); they're polite and professional, but they don't have a lot of sympathy for me.
In all of the threads I read on discussion forums, I see a recurring theme: "you don't need to have antivirus software on your Mac". I don't violently disagree with that assessment, unless your system configuration is governed by an enterprise security team with an anti-virus software requirement.
For those of us in that boat, I'd like to see the recommendations of others that have actual experience. I would really be jazzed if someone from Apple would comment. Worried that I'm going to hear crickets. Have a great day!
Sophos is normally quite well-behaved, and should not cause the kinds of problems you describe. I have tested it at length on a 2010 MacBook Pro with 2.53 GHz i5 processor, and haven't seen any performance issues. Are you using the free Sophos Anti-Virus for Mac Home Edition, or something else?
If you're using the free version, and you're positive that these performance issues correlate exactly with Sophos downloading updates every single time, then you should try asking over on the Sophos for Mac forums:
Regarding the necessity for anti-virus software, people who say you don't need it are mostly right, but for the wrong reasons. They poo-pooh the idea of malware on the Mac and make misleading statements like “There are no Mac viruses”. But, you really don't need anti-virus software, for the most part. You're in one of those situations outside "the most part" because you're required to use it by your employer. Thus, the question of whether you need it is rather moot, and the only question you need answered is what anti-virus software to use. Sophos is one that I generally recommend most highly, so if you can find a way to solve your issue rather than switching to something else, that would be best. (Note that I have absolutely no affiliation with Sophos.)
If you want to learn more on this topic, see my Mac Malware Guide.