News that 600,000 Macs have been found to be infected by a trojan horse is beginning to raise doubts about the vaunted invulnerability of Apple‘s (NASDAQ:AAPL) Mac OSX operating system to viruses and other malware. PCWorld says that more than half of Macs infected with the Flashback trojan are based in the U.S., including 274 in Cupertino, Calif. — Apple’s hometown.

The isn’t the first time that the Mac platform has been hit. Last year’s MAC Defender garnered some attention, including countermeasures from Apple, but never really grabbed headlines the way the Flashback trojan has.

In its last quarter, Apple sold 5.2 million Macs (up 26% on the year), in a market where world PC sales were at their worst growth levels since 2001 and Hewlett-Packard (NYSE:HPQ) — the world’s top PC vendor — saw a near 16% decline in its computer shipments.

A presumption of safety

Mac sales are doing so well partially because of the so-called “halo effect” where iPhone and iPad users jump on board the Apple bandwagon, but also because of Macs’ growing acceptance in business environments. A Wired article puts Apple’s penetration rate in U.S. businesses at 10% of PCs. A big part of that appeal has been the perception that Macs aren’t susceptible to the flood of malware that hits Windows PCs every day. This is a point that Apple pushes in its marketing, proudly announcing (on its “Why You’ll Love a Mac” page):

“It doesn’t get PC viruses. A Mac isn’t susceptible to the thousands of viruses plaguing windows-based computers. That’s thanks to built-in defenses in Mac OS X that keep you safe, without any work on your part.”

Tech insiders tended to agree with Apple’s assessment. For example, in CNN’s Apple 2.0 column, even as the 2011 MAC Defender threat was being described, Mac antivirus programs were being dismissed as “more trouble than they’re worth.”

The ongoing debate has been whether OSX is inherently more secure, or whether the relatively low numbers of Macs make creating and distributing Mac-specific malware more trouble than it’s worth; if the answer turns out to have been the latter, increasing market share could spell trouble for the platform.

The beginning of an onslaught?

Apple has since released a pair of system upgrades designed to block the Flashback Trojan, so the immediate danger should be over for now. But in some ways, this is beginning to look a bit like 1996, when Microsoft (NASDAQ:MSFT) began to face an increasing virus threat to its Windows95 operating system in the start of a relentless series of attacks that would see a Windows retail environment where six of the top-10-selling applications were antivirus or security packages.

While it is possible that Flashback could cause some IT departments to rethink their newfound Mac-friendly attitude, the iOS halo effect would still be present. And besides, Apple is so post-PC these days that slowing Mac sales would have little effect on its bottom line. Computer security software publishers like McAfee (NASDAQ:INTC), Symantec (NASDAQ:SYMC), Kaspersky Lab, and Trend Micro, on the other hand, could began to see demand for their products increase rapidly.

The halo effect and security services

If Mac owners and IT departments that have begun to adopt the Mac platform get nervous, the antivirus companies could have an opportunity to cash in on millions of computers that have previously ignored or under-deployed their products. Cult of Mac goes so far as to say that “business Macs that don’t include centrally managed antivirus protection may be ticking time bombs.”

Symantec’s Norton AntiVirus for Mac sells for $49.99. It can’t be selling many copies of the Mac version, but if Apple were to recommend that its customers install antivirus software, those five million Macs sold per quarter could be up for grabs — not to mention the installed base of iMacs, MacBooks, MacBook Pros, Mac Minis, and Mac Pros.

That potential would represent a significant boost to Symantec’s $6.2 billion in revenue for 2011 and offer a potential windfall for all security software publishers who offer Mac products. Depending on how the Flashback trojan episode plays out, we could be looking at the start of another Apple halo effect — the push to install professional antivirus software on a generation of Macs once considered invulnerable.