By Ian Bush

PHILADELPHIA (CBS) - Mac owners aren’t used to dealing with viruses. But that’s changing, now, with the biggest-ever threat, affecting hundreds of thousands of Apple computers.

It’s malware called ‘Flashback.’

“It’s not a worm that spreads by itself,” explained Pierre-Marc Bureau.

Pierre-Marc Bureau, a security intelligence program manager with the anti-virus firm ESET, says you can get it by browsing to a malicious website.

It works by exploiting a vulnerability in Java. Oracle has released a patch, but Apple hasn’t ported it for use with its operating system.

“One component of the malware keeps the door open for attackers to do whatever they want with your computer.”

But so far, Bureau says, it hasn’t shown any keystroke-logging capability. Instead, it’s sending details on your web-surfing habits to a server — maybe to sell to advertisers.

Macs traditionally haven’t been a prime target for computer criminals, but that’s changing as Apple grows above and beyond the PC industry.

Bureau says it doesn’t matter which computer you use: everyone is at risk.

“If you have good habits — for example, keeping your computer up-to-date, or using security software — then you’re more protected than by your choice of operating system,” he says.

The anti-virus firm Kaspersky Lab has built a site where Mac users can easily see whether they’ve been infected — and to clean your machine.