A trojan is a virus please correct me if I'm wrong
There's a lot of needless arguing and misinformation going on here, ironically in response to an issue that isn't actually caused by malware at all.
First, let's get terminology straight. "Malware" is a catch-all term for all malicious software. Trojans, viruses, worms, rootkits, etc all fall into this category. This is the best term these days, as many malicious programs don't actually fit perfectly in any of those sub-categories.
A "trojan" is a program that tricks the user into installing it, and cannot get installed by itself. Most Mac malware lies firmly within this category. A "virus" is a self-replicating program that attaches itself to another program in order to infect new systems. There is no Mac malware that fits that definition. A "worm" is similar to a virus, except that it does not need to attach itself to anything, and there are also no Mac worms. A "rootkit" is any kind of malware capable of illicitly attaining root-level permissions... method of installation is irrelevant.
So, technically speaking, it is true that there are no Mac viruses. On the other hand, most people do not properly understand what these definitions mean, and they often interpret the word "virus" to mean what the word "malware" does. Thus, I see no value, and much potential harm, in stating that there are no Mac viruses.
Regarding recent Java- or Flash-related outbreaks, such as Flashback, the malware involved in those cases does not fit strictly in any category. Malware that sneaks in through vulnerabilities in third-party software (like Java or Flash) is not a trojan, since it does not require the user's assistance to install itself. However, it is also not a virus or worm, as it is not self-replicating. Users must visit a compromised web site in order to become infected. This malware also would not usually be called a rootkit, as most of the time root access is not obtained, or even sought out.
For more information on this sort of thing, see my Mac Malware Guide, which Ana referred to earlier.
