SAN JOSE -- Benjamin Daines was browsing the Web when he clicked on a series of links promising to deliver pictures of an unreleased update to a computer operating system.

Instead, a window opened on his screen and showed strange commands being run, as if the computer was under the control of someone -- or something -- else.

Daines was the victim of a computer virus.

Such headaches are hardly unusual for computer users that run Microsoft Corp.'s Windows operating system. Daines, however, was using a Mac -- a computer often touted as being more secure and immune to such risks.

He and at least one other person who clicked on the links were infected by what security experts call the first ever virus for OS X, the operating system that has shipped with every Mac sold since 2001.

After surviving unscathed from the onslaught of viruses and other computer malware unleashed on the Internet over the past decade, Mac aficionados can no longer take for granted they are immune to such attacks, security experts said.

"It just shows people that no matter what kind of computer you use you are still open to some level of attack," said Daines, a 29-year-old British chemical engineer. He's one of those who considered Macs invulnerable to such attacks.

Apple's increasingly iconic image, growing market share and adoption of the same type of microprocessors used in machines running Microsoft Windows are making the Apple machines a bigger target, they warn.

But Bud Tribble, Apple's senior vice president of software technology, disputed claims that OS X is more susceptible to attack now than in the past.

"It's become less vulnerable as we've continued to improve security," he said.

In Daines' infection, a bug in the virus' code prevented it from doing much damage. Still, several of his operating system files were deleted, several new files were created and several applications, including a program for recording audio, were crippled.

Behind the scenes, the virus also managed to hijack his instant messaging program so the file was blasted to 10 people on his buddy list.

"A lot of Mac users are in denial and have blinders on that say, 'nothing is ever going to get to us,"' said Neil Fryer, a computer security consultant who works for an international financial institution in Great Britain. "I can't say I agree with them."

Fryer, himself a Mac user, said over the past year he has begun taking additional precautions to make sure he doesn't fall victim to an attack. He spends more time than in the past scrutinizing his security logs for signs of intruders, and he uses a firewall and additional security applications, just as he would with a Windows-based machine.

Among the other signs Macs are a growing target:

-- The SANS Institute, a computer-security organization in Bethesda, Md., added OS X to its 2005 list of the 20 top Internet vulnerabilities. It was the first time the Apple operating system has been included since the experts started compiling the list in 2000.

-- The number of discovered Mac vulnerabilities has soared in recent years, with 81 found last year, up from 46 in 2004 and 27 in 2003, according to the Open Source Vulnerability Database, which is maintained by a nonprofit group that tracks security vulnerabilities on many different hardware and software platforms.

-- Less than a week after Daines was attacked in mid-February, a 25-year-old computer security researcher released three benign Mac-based worms to prove a serious vulnerability in OS X could be exploited. Apple asked the man, Kevin Finisterre, to hold off publishing the code until it could patch the flaw.

The Mac's vulnerability could also increase as a result of Apple's transition to a product line that uses microprocessors made by Intel Corp., security experts said.

With new Macs running the same processor that powers Windows-based machines, far more people will know how to exploit weaknesses in Apple machines than in the past, when they ran on the PowerPC chips made by IBM Corp. and Motorola Corp. spinoff Freescale Semiconductor Inc.

"They have eliminated their genetic diversity," said independent security consultant Rodney Thayer. "The fear is that we're going to run into a new class of attacks."

Apple's Tribble noted that OS X was derived from FreeBSD, open source software that was built from the ground up to provide security for computers networked together. Since its origins in the early 1990s, the Unix-based FreeBSD has continually been battle-tested by college students and computer security specialists, whose discoveries of holes have allowed security to improve over time.

OS X, Tribble said, is designed to be Internet safe out of the box, without the need for firewalls or additional security software. He praised OS X for making it easy for users to automatically install security patches.

He also disagreed with people who say Intel processors make Macs a bigger target.

"All the things we've been doing to make Mac OS X secure continue to be relevant on Intel," he said.

To be fair, real-world attacks of Macs remain virtually nonexistent, and Apple gets high marks from many security experts for making an operating system that, by default, resists viruses and other malicious attacks. F-Secure Corp., which has been providing security software since 1990, discontinued a version of its antivirus program for the Mac in the late 1990s.

"There was no market because there were no Mac viruses," Mikko Hypponen, F-Secure's chief research officer, explained.

Windows, by contrast, often requires users to modify a number of default settings and to install antivirus and antispyware programs to lock down a machine.

For instance, Windows automatically gives an end user full authority to format a hard drive, delete operating system files and carry out other administrative tasks often central to the successful execution of malicious programs. By default those types of commands require OS X users to enter a password first.

But as Daines can attest, such safeguards are no guarantee. For reasons he still doesn't understand, the virus that attacked his PowerBook G4 was able to elude OS X's password protection, possibly, he said, because he was already running in administrator mode.

"We're all sort of waiting with bated breath to see if any problem will happen and the jury is still out," said Thayer, the independent security consultant. "I don't think you'll find a consensus."