I am a bit confused here. I have very little knowledge about viruses, worms and stuff and Memnochs explaination is.. uumm .. well "abracadabra" to me

No seriously; I can understand a little of it, but what I want to know is: Does someone send these viruses ON PURPOSE or could it be someone whose computer is infected with the virus but he or she doesn't know about this. And who is visiting IW and therebye accidently sending these viruses? I mean could it be me? *looks around very, very innocently* I had my dad scan our computer and he said nothing was wrong so I hope I am not the one who's causing the problem.
Please respond guys, I really want to know.