Whereas you know for sure that you are not
That is correct.
If I have a cold then I feel unwell and start coughing.
If I have flu, then I weel unwell and start shivering and have aching limbs and start coughing.
This is a common misconception about (computer) viruses - people automatically assume that if their computer is affected, then they will notice something out of the ordinary. Well, the only time that you will do that is when you acquire a virus that is designed to be activated by a trigger of some sort, which will flash a message of some sort, encouraging (or even panicking) you into buying security software from them.
There is another type of virus - it sits there on your computer, and you are completely oblivious to it, whilst in the background it can be recording everything you type and uploading it to somewhere on the other side of the world, or using your email connection to send out hundreds of junk emails. You will be completely oblivious to what's going on. Some viruses will even take pictures from your webcam and upload them, so be careful what you wear in front of your PC, because someone may be watching
There was a virus doing the rounds about 10 years ago that was incredibly tiny (in terms of program size) called
Back Orifice (try Googling for it) - when the victim was infected with this virus, they could be found by scanning IP addresses with a program, and once they were found, the person at the other end could see a picture of their desktop, examine their entire disk drive contents, download files from their hard drive, upload files to their hard drive, rename files, delete files, see which programs they were running, take a picture with their web cam and download it, open the CD door, and make their PC reboot.
My point is - if they could do that 10 years ago, when we were all on dial-up, what can they do now, with continuous broadband connections ?
I wish I had £1 for every time I heard someone say (or read) "I'm not using a virus checker, and I don't have any problems".
My response (as always is) "What problems were you expecting ?".