“Can you get spyware or malware on mac without downloading anything?”

No, you can’t. It’s not impossible, but it is highly improbable and would have to come through an unpatched flaw in your web browser, Flash plugin, or other plugin connected to the network. If you’re really that paranoid about it, you should use Firefox with the NoScript extension and turn off all network services in OS X’s System Preferences.

Unlike Windows, Mac has proper privilege level separation. So even when you have a so-called “administrator” account, you’re still operating as a limited user almost all of the time, and you have to password-authenticate to temporarily gain root privileges to make system-wide changes.

In Windows, you’re total administrator by default. You can secure Windows by using a limited user account, but Microsoft hasn’t made it easy to operate as limited user full-time, since the “Run as…” feature is crippled in Windows.

You’ll see that almost all Mac malware either requires user intervention (trojans, for example) or an unpatched system.

It doesn’t mean Macs are invincible. It means Macs have a pretty strong default security, and it means you still shouldn’t do anything stupid.

“Also do some sites just say that you have spyware or malware just to get you to download something?”

Definitely. And, as you can see from all the Yahoo! Answers users trying to get rid of “Antivirus 360″ and “Antivirus 2009,” clearly that strategy works.

“Also how can you tell if mac is infected?”

Well, there’s no way to know 100%, just as there isn’t in Windows. Clever malware writers design malware to be undetectable and stealthy. Stupid malware writers write malware to slow down your computer or bombard it with pop-ups.

Again, if you’re really that paranoid, then reinstall OS X every week. Reinstalling is the only way to be 100% sure no malware is on your computer.

I guess it’s also possible that I and everyone I know could be struck by lightning in the next month, but I’m not going to lose any sleep over it.

Spyware designed for Windows is ineffective against the Mac. Hackers
have begun working different angles to ensnare Mac users, whose
confidence in the Mac OS may have lulled them into a false sense of
security. For a long time, Mac users were insulated from attacks. The
reasons for this were two fold:

1. The Mac operating system was designed to be much more
secure than the traditional Windows system. This combined with
the fact that

2. the number of Mac users relative to PC users was incredibly
small.

There was little incentive for hackers to spend the time and effort to
build malware that could breach the Mac’s security. There is also the
little issue of corporate reputation. Apple has long been considered the
scrappy underdog to Microsoft’s industry dominance. Hackers have
long taken great pleasure at pounding holes in Windows, Internet
Explorer, and the Office Suite of products.

But as Macs have become more widespread, and Apple’s reputation as
an innovator and leading-edge design company, many of the barriers
keeping hackers at bay have weakened. Hackers now employ two
effective strategies to spy on Mac users.

The first strategy is phishing. Phishing is an incredibly effective tool hackers and
identity thieves use to steal personal information, and it takes little
effort to install spyware on a Mac if the user offers up their information
willingly.

The second strategy hackers employ is through unknown downloads.
With the plethora of mac applications being created, there is little
doubt that the unscrupulous have been busily creating apps which
could compromise a Mac’s security. For example, spyware could be
part of a song download or as a browser plug-in. As soon as the user
enters the password (Macs require a password before any new
software installation), attacks could begin.

As with PC spyware programs, the key to the ones designed for Macs
is research and more research. Prevention, detection and removal are
the most important features users should be looking for. But the best
defense remains vigilance. Users need to exercise caution when
logging onto unfamiliar sites and downloading content that a cautious
person would deem questionable.

Simply relying on Mac’s well-earned reputation for security is no longer
enough.