NIMDA does not infect the MAC! But …
(email protected) is a very powerful virus that can be defined as a "mass-mailing worm" that uses different ways to spread: the Worm spreads automatically via e-mail, searches for open network connections, tries to copy itself on Microsoft Server IIS type and infects both local files and files on connected remote computers. In practice, it exploits a weakness of the software for Microsoft networks that the virus programmers have been able to identify through Unicode Web Traversal.
The worm spreads through e-mail by sending a message that contains an attachment called "README.EXE" in MIME format that is executed by reading or previewing the file. A patch to solve the problem can be found at the Microsoft site on this page.
But the greatest danger lies in the news of the Worm's propagation via the web: the web servers based on Microsoft software that are affected offer the site visitor that they are invited to download an .eml file (of Outlook Express) that through MIME mode propagates as in email mode. Users should disable downloading of archives in internet security options.
In addition to this, if a computer belonging to a local Windows network is infected, the virus can spread to other connected computers by creating an account with Administrator privileges on its own.
Updated definitions of the software to eradicate the virus are already available on the Symantec website.
In general we can recommend you not to download or read archives that have arrived via email which the sender is not familiar with or that have dubious content. The use of an alternative email client or maybe Mac OS (:-)) would be desirable because it would also protect you from other viruses, but in general a prudent behavior can prevent many troubles for your computer and now also for the Windows computers on the network.