do i need antivirus software for my apple mac

Not really. See:

Helpful Links Regarding Flashback Trojan and Virus Protection

An excellent link to read is Tom Reed's Mac Malware Guide.

A link to a great User Tip about the trojan: Flashback Trojan User Tip.

To check for the trojan: Anti Flashback Trojan 2.0.4.

A Google search can reveal a variety of alternatives on how the remove the trojan should your computer get infected. This can get you started. Or the preferred method is to use Apple's protection tool: Flashback Malware Removal Tool 1.0.

Or, open Software Update. If you do not have the Apple protection software installed it will download and install it via Software Update. If no update appears that means you either already have it installed or it isn't needed for your system. The software is only available for Leopard, Snow Leopard, and Lion versions of OS X.

Also see Apple's article About Flashback malware.

For general anti-virus protection I only recommend using ClamXav.

For the DNS Changer malware see the following:

Will your internet service cut off on July 9?

DCWG | DNS Changer Working Group

DNS Changer Malware- What to Do If Your Computer's Infected

Mac OS X versions 10.6.7 and later have built-in detection of known Mac malware in downloaded files. The recognition database is automatically updated once a day; however, you shouldn't rely on it, because the attackers are always at least a day ahead of the defenders. In most cases, there’s no benefit from any other automated protection against malware.

The most effective defense against malware is your own intelligence. All known malware on the Internet that affects a fully-updated installation of Mac OS X 10.6 or later takes the form of trojans, which can only work if the victim is duped into running them. If you're smarter than the malware attacker thinks you are, you won't be duped. That means, primarily, that you never install software from an untrustworthy source. How do you know a source is untrustworthy?

1. Any website that prompts you to install a “codec,” “plug-in,” or “certificate” that comes from that same site, or an unknown site, merely in order to use the site, is untrustworthy.
2. A web operator who tells you that you have a “virus,” or that anything else is wrong with your computer, or that you have won a prize in a contest you never entered, is trying to commit a crime with you as the victim.
3. “Cracked” versions of commercial software downloaded from a bittorrent are likely to be infected.
4. Software with a corporate brand, such as Adobe Flash Player, must be downloaded directly from the developer’s website. No intermediary is acceptable.

Disable Java (not JavaScript) in your web browser(s). Few websites have Java content nowadays, so you won’t be missing much. This setting is mandatory in Mac OS X 10.5.8 or earlier, because Java in those versions has bugs that make it unsafe to use on the Internet. Those bugs will probably never be fixed, because those older operating systems are no longer being maintained by Apple. Migrate to a newer version of the Mac OS as soon as you can.

Follow these guidelines, and you’ll be as safe from malware as you can reasonably be.

Never install any commercial "anti-virus" products for the Mac, as they all do more harm than good. If you need to be able to detect Windows malware in your files, use the free software ClamXav — nothing else.

Thanks for that advice! It is helpful.