PC Magazine has an interesting article and slide show on 5 Fast Spreading Virues that are active today - and how one can find themselves exposed to these viruses.
Oh, and if you're wondering why people do this? Well, some will do this for the same reason that someone will decide to vandalize a car, a building, a work of art. Others will do this to demonstrate how smart or clever they are. But the majority of these things are done to make money as easily as possible - via identity theft or leveraging other ways of making money on the Internet.
As PC Magazine noted in the above link...
Operation Ghost Click, the largest cybercrime ring to date, was just brought down by international law enforcement efforts. Its unraveling made clear that just seven people in Russia and Estonia for a payout of $14 million could infect 4 million computers. Victims—from home users to NASA—had their browsers hijacked so that when they clicked on what they thought were legitimate web pages or ads, they instead ended up on pages that netted the criminals cash.