It's called a fly by or drive by virus. Someone has a hidden macro embedded in the page that initiates the download action just by going there. You are unaware of this until the virus shows up.
NoScript is a good plug in to deal with this sort of thing. It blocks all scripts until you allow them. Of course, you will have to allow scripts for sites to work, like YouTube when you go there. The trade off is that you have a measure of protection against hidden nasties.