Virus' don't help computers, they're given the name virus because they harm your computer, my suggestion, don't get a virus.
A virus
Not to all computers
a virus that is affecting many computers just get protection
Yes, computers are always consistent unless they have a virus or bug.
The impact of IT in our society is that the IT maintenance workers help us. What that means is that they are the ones who fix our computers or reboot them if your computer had a major virus.
The Elk Cloner virus is very old, designed to run on Apple II computers. It cannot infect modern computers.
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
Yes. Delete System32 now.
it may spread virus
Viruses are designed to harm the computer. These no such thing as a virus that does not cause harm to computers. If your virus scanner detected a virus, the damage is already done.
A Benign virus (when referring to computers) is a virus that causes no damage. It can display a random message or make a sound on some specific ocassion.