the effect of having a virus on your computer depends on which virus you have. some view your personal data like credit cards and stuff. some destroy your computer so it wouldn't work anymore and you would have to reinstall windows and all your data could be lost.
Virus' don't help computers, they're given the name virus because they harm your computer, my suggestion, don't get a virus.
A program that is designed to cause damage to computer files.
A virus
Not to all computers
None, unless you back up your facebook contacts list to your backup drive or they give you a computer virus.
a virus that is affecting many computers just get protection
Yes, computers are always consistent unless they have a virus or bug.
The Melissa virus is a mass-mailing macro virus. It can effect email that is sent and infect the computer when the email is opened.
The Elk Cloner virus is very old, designed to run on Apple II computers. It cannot infect modern computers.
useful effect viruses
Windows computers are the target of almost every computer virus on the planet because Windows is the most popular type of OS (Operating System). Think about it, if almost 94% of the population who have computers on Earth are using Windows, don't you think hackers and virus writers will take advantage of that and aim at every Windows computer? Apple computers and like you stated, Unix computers, are less vulnerable to virus attacks because the majority of computers are Windows (that's not to say you still can't get a virus if you have Apple or Unix).
Stealth virus