So my mate has a Mac with no security systems installed. All I know is that I've been getting a lot of spam emails from his account that he doesn't know about, but he reckons he doesn't need any of that kind of thing because it's a Mac. I know nothing about computers (never owned a Mac to begin with) but hopefully you lot could enlighten me/us. I'm pretty sure he still needs to install some kind of protection.

Mac's are a lot less likely to get viruses because not as many exist for them, there are a number that are possible but I don't think they are prevalent enough to warrant bothering with antivirus. I would assume a more likely cause for them getting their account hijacked is using the same password across sites or falling for scams.

You don't need an anti-virus to prevent spam-email; you need a spam-email filter. What email provider does he use? Some providers have better spam filters than others. As long as he doesn't open or reply to any of the emails it shouldn't be a problem for him.

Most malware is moving cross-platform. Mac OS X malware has been on the rise lately, and is projected to continue rising. The biggest problem with Mac OS X is both the users and Apple. The users think they are immune, which helps the malware spread faster, and Apple is completely incompetent at releasing security patches.

http://www.sophos.com/en-us/products/free-tools/sophos-antivirus-for-mac-home-edition.aspx

That said, as the others have mentioned, spam isn't virus-related. Also, if using webmail, there's nothing your security suite can do to stop the spam. Spam filters only work on local clients like Thunderbird and others.

Thank you everyone for your info - I'll relate it to him and hopefully he'll sort it out.