
There are a lot of ways to spread a computer virus but the most common method is by hiding the malware inside an innocuous-seeming file such as an emailed greeting card or file-shared music download and I'd bet dollars to disc drives that at least 75% of all blinking "click me" web ads contain spyware or worse.
You never see web ads that explicitly state : "Click Here to get a Trojan that will hijack your computer and turn it into a Zombie Web-Slave until it crashes and burns and you have nineteen nervous breakdowns before you finally give up and buy a new computer."
That would be too easy. In reality the virus pretends to be something else.
If I were an Evil Spirit, I wouldn't claim to be one, instead I'd masquerade as a Man of God and use that charade as leverage to gain access to - and infect- as many willing victims as possible. In my ideal scenario, I'd get to physically lay hands on a person or persons who wield considerable real-life power. I would control them and take their power and combine it with my own, converting it to Dark Side Force and using it to influence weak minds and achieve my self-serving goals.
Like, first I'd possess a small-town mayor and use my demonic powers to push them, against the odds, to the top of their State government. That would get boring pretty quickly though, so I'd wait until the next National election and then I would use my Sith abilities to reach out and trick the most easily-swayed of the two Presidential candidates into choosing me as their vice-presidential running mate, thus giving me a fast-track to the White House. During the campaign, I'd leech the energy out of my partner, hastening their demise in hope that I might ascend more quickly...if I were a real bitch, I'd put a little arsenic in his daily tea just to be on the safe side.
My partner would live just long enough to get elected and then I'd take over. I'd have meetings with the most prominent Evangelical leaders- most of whom are already double-agents in the war between Light and Dark anyway- I'd bend these leaders to my will and force them to use their pulpits to spread my diseased message of hate, endless war and violent intolerance.
People love that stuff, ya know...I swear, sometimes it seems as if most folk literally cannot tell right from wrong. If I were an Evil Spirit, I'd use that weakness to my advantage and convince good people to do bad things and push Bad People into doing the unspeakable.

Remember Susan Smith? She drowned her children and blamed their deaths on an imaginary Negro. Lovely woman, but not available at the moment.

How about this lass? She beat herself up and blamed her self-inflicted injuries on an imaginary Negro who also happened to be an imaginary Obama supporter, hence the 'B' etched on her cheek. Somebody shoulda told this stupid piglet that letters as seen in mirrors are backwards...on second thought, nah, no one shoulda told her that.
It's not inconceivable that the actions of either woman could have lead to the false arrest, conviction and execution of an innocent, non-imaginary African-American. Not that that sort of thing could ever happen in America. Just ask Troy Davis. Don't know who Troy Davis is? Find out.
Fortunately, I'm not an Evil Spirit. Unfortunately, I don't have to be. Folk seem to be doing just fine at being rotten on their own.
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*!Ring! !Ring!*
Me: Hello?
Caller: Hi. My name is Alice and I'm calling on behalf of John McCain...
Me: I'm sorry for your loss.
Caller (pause): Uh, we haven't lost. That's why I'm calling, we need...
Me: No. I meant your mind. You've lost it if you think I'm voting for McCain.
*click*