The sky began to fall October 22, thanks to this discussion on MacInTouch. Readers learned that the first ever virus for Mac OS X had been found. Again!

No, wait... the sky is not falling. If you read the reaction to the story on Macintouch you discover that "Opener" (aka Renepo) is not a worm or a virus. It's a script that can do quite a bit of mischief if it runs on an OS X system that has already been compromised.

Here's a pithy summary from anti-virus maker Sophos:
Note that any attacker trying to plant this worm in your network would need to get root access on one of your boxes first, meaning that you would already be "owned".

In other words: this is a root kit, not a virus or a worm.
Nothing to see here. Move along, please.

What's the difference between a root kit, a worm and a virus? Don't ask CNET or ZDNet. Their headlines included the words "worm" and "virus" in the first drafts; after some fact-checking, they switched to "malware" -- a handy catch-all that's often used to describe various security threats.

One Macintouch reader got it right:
"This is not a virus, not a worm, and not even a trojan (a "trojan" is something that masquerades as one thing and does something else, usually undesirable; this script does exactly what it advertises...for example, a "trojan" would be some other installer that would also secretly put this script on your computer).

Additionally, there is no way for this to spread or propagate in any automated fashion, making it completely worthless. The only reason this script is getting any attention at all is because it is targeted specifically at Mac OS X, and does Mac OS X-specific things; but at its heart, it's nothing more than a UNIX shell script - one that needs to be manually installed by someone with admin/root or physical access to the machine!
- Dave Schroeder, posting on Macintouch.com

Slashdot weighs in
Readers on Slashdot offered several bits of good advice, including this:

Hey! Mac developers! Quit requiring privileged steps during install! Seriously. The Mac app architecture is designed so you can put all your files into a single bundle without littering crap all over the user's system folders. I, for one, tend to kill any install that asks for my admin password (which is why I'm still using Preview instead of Adobe Acrobat).

and this:
The best fix for this problem is to apply common sense. Do not give your admin password to any application except an installer for software acquired from a trusted source, or the OS X system utilities.

and this:
An inherently secure design is one in which there are no APIs that depend on the ability to perform trusted operations from potentially untrusted objects. The MS HTML control, for example, depends on that ability for a document in the most trusted zone to launch arbitrary code without restructions. That means that if an attacker can get any application (ANY application that uses the HTML control) to open a document that's in that zone, it's in.

The final word goes to inkswamp (Score: 5, Insightful):
"OS X virus" is the new "Apple is dying"