Information security companies such as Symantec, Sophos, and McAfee have identified multiple security concerns related to the Apple Computer Macintosh running OS X, including two worms written by unknown hackers. The worms were meant to replicate themselves on other Macs through Apple's iChat instant messaging (IM) software and Bluetooth wireless communications ability. A German graduate also has discovered a vulnerability in OS X that could allow a hacker to install possibly damaging code on a Mac through the Safari Web browser. The worms were not as fearsome as those that attack Windows computers, and security experts called them proof of concept programs that required users to go through several steps on their computers before being infected. Alfred Huger, senior director of engineering for Symantec's security response operation, is predicting 'that there will be a gradual erosion of the idea that Macs are a safer operating system (OS) than Windows. In comparison to Windows machines, Macs have been a small market and did not attract many hackers. Apple, with its sales of 4.7 million Macs in calendar 2005 (a 35% gain from the 3.5 million sold in 2004 and much higher than the 16% growth for the PC industry) has gained more visibility, which was also greatly increased by iPod sales. Microsoft has patched many previous holes in its Microsoft Windows XP and is also promoting Windows Vista, the next versions of the Microsoft OS, as having much better PC security.