1. I have seen too many to count around here on DA, once you click the link it downloads it self to your computer. 2. as I said before, it downloads its self to your computer, and hides in your files. 3. not always, and yes ads can have hidden viruses in them. you can find out about it if you read the Deviantart News.[link] 4. some people can't afford the pricy stuff. the free stuff works just as good if you have a ad blocker.
also most anti-spyware scanners tell you where you picked the virus up.
To add on to the fun, why click ads? and why click unknown links? hover the mouse above the link, and atleast in chrome i can see where he link goes, i don't click it unless i now the one sending it and the site it sends me to