Apple is fighting what security experts say may be the most pernicious types of computer virus to ever target its line of Mac computers.

The consumer-electronics giant issued a warning to customers about a recent scam that tricks users into downloading a nasty malicious software program.

The malware, called MacGuard, falsely tells the user that their computer is infected with a virus and to remedy it, users should enter their credit cards and other personal information.

What makes it especially lethal is the ease with which it can creep into the user environment.

Unlike the previous variants of this fake antivirus, no administrator's password is required to install this program, explained security firm Intego. Since any user with an administrator's account - the default if there is just one user on a Mac - can install software in the Applications folder, a password is not needed.

It is one of the first major campaigns that hackers have launched against Mac users.

To date, criminals have focused on writing malicious software for machines running Microsoft Corp's Windows operating system, which inhabits more than nine of every 10 personal computers.

But as Macs have grown in number, they have become more attractive targets.

The virus infects Mac computers through Web sites that tell users to install antivirus software.

To fix the problem, Apple posted a list of instructions on its Web site Wednesday telling users how to remove the intrusive software from Macs.

The company also said it planned to offer a software update that would completely eliminate the virus in the coming days.