Among other features, the software will remove worms and viruses that affect only PC users from Mac e-mail to prevent the user from passing malicious files on.

Symantec (NSDQ: SYMC) on Tuesday released Norton AntiVirus 11 for Mac, a new version of the company's antivirus software that's compatible with Mac OS X 10.5, also known as Leopard.

Symantec said its security software features a redesigned user interface and better performance.

Among the improvements, Norton AntiVirus 11 for Mac ($50) includes a signature-based protection layer designed to block malware and viruses that might be acquired as a result of downloading images, music and software. Such attacks, which occur at the Web application layer, represent 78% of exploit attempts, according to Symantec.

The software will remove worms and viruses that affect only PC users from Mac e-mail in order to prevent the user from inadvertently passing malicious files on. It has been engineered to have less of an impact on system startup time and to perform updates more quickly in the background, without interrupting other computing activities.

Norton AntiVirus 11 for Mac can be accessed via the command line interface in the Terminal program that comes with Mac OS X. This will appeal mainly to advanced users who want to access antivirus scans and other features using their own scripts.

Apple's computers have long been overlooked by malware writers, who tend to craft threats for maximum possible penetration. As a consequence, security applications haven't really been must-have items, as they have been on Windows PCs.

But as many security companies have observed recently, perhaps a bit gleefully, that's starting to change. Apple's growing share of the PC market, the success of its iPhone, and the popularity of iTunes on both Windows and Mac computers has prompted at least one prominent malware group to being crafting exploits that affect Macs. A malware program known as TrojanDNSChanger, which exists in both Windows and Mac OS X variants, represents one example of this trend.

Apple appears to be busier this year fixing security flaws than it has been in the past. In 2007 so far, it has issued 32 security-related updates, compared to 22 in 2006 and 23 in 2005. However, this year's larger number of security updates may be due, in whole or in part, to the addition of new products like the iPhone and Apple TV.

Mike Romo, a product manager for the Mac team at Symantec acknowledges that he often receives questions about whether Mac users really need a security program. Romo said he used to work as an IT administrator in a company that used Macs and for years went without antivirus protection. But he said that the threat landscape is changing.

"It's a little bit of insurance in a rapidly changing environment," Romo said, noting that the software is also useful as a way to prevent the retransmission of Windows malware, which can get e-mail blocked or quarantined.

And looking ahead, Romo predicted that Mac users will be more likely to be running multiple operating systems, thanks to Apple's Boot Camp and virtualization software from Parallels and VMware. In such cases, he said that Symantec's software would be particularly helpful.

Some day soon, if not today, Mac users may actually have to start thinking about security. With the arrival of Norton AntiVirus 11 for Mac, Symantec appears to be ready for that day.