MacDefender is not a virus. It's a trojan AND an IQ test. How do you get MacDefender? By being stupid. Here are the stupid things you have to do in order to get SUCKERED by MacDefender.
1. Go to a website of ill repute.2. Believe the redirected website that claims your Mac is infected by a virus. (hint: it's not)
3. Have your browser set to automatically open "safe" files.
4. Click on "continue" when the MacDefender installer automatically opens.
5. Enter your administrator's password to install MacDefender to remove the virus that is not really there. (YOU are doing all of this).
6. Really believe that your Mac is infected with a virus because porn websites are automatically opening up.
7. Enter your credit card number into MacDefender. Everything above is a choice YOU have to make. You can stop at any time through # 6. Once you complete # 7, MacDefender also revokes all cool points, self-esteem, degrees conferred, etc. You are officially stupid. Just because it is written down doesn't mean it's true. Think for yourself. One more thing… After giving MacDefender your credit card number, Steve Jobs shows up at your door step with an Army of Apple Geniuses. They take away all of your Apple gear. And leave you with a Dell computer running Windows (you're not smart enough for Linux), a Zune, and a Blackberry. The Unofficial Apple Weblog has an excellent article on MacDefender and how to avoid it, or get rid of it. Read it. You'll be smarter for it.
http://www.tuaw.com/2011/05/19/macdefender-malware-protection-and-removal-guide/