The Michelangelo virus was expected to create a digital apocalypse, infecting millions of computers and had their information wiped, from the quote said by John McAfee. Later assessments of the damage showed the aftermath to be minimal. When McAfee was asked how he came up with that number, he said that he had estimated a range from 5 thousand to 5 million, but the media naturally went with just the higher number.
The virus was first discovered in February, 1991 in Australia, and first came to widespread international attention in January 1992 when some computer and software manufacturers had accidentally shipped products infected with it. Designed to infect computers with DOS, the virus was like any other boot sector virus that operates at the BIOS level. The virus is dormant most of the time until March 6th, the birthday date of the artist Michelangelo. Despite there is no reference to the artist in the virus, the name and date of activation have made it very likely that the artist is the reference to it.
At the birth date of the famous artist, if the computer is an AT or a PS/2, the virus overwrites the first one hundred sectors of the hard disk with null and random characters, rendering the data irretrievable for average users. The virus also moves the computers Master Boot Record and original boot sector to another place.
The Michelangelo virus is similar to the original Stoned virus. It also becomes a memory resident at the top of the system memory. It's ability to overwrite data can make recovery unlikely, if not impossible. The virus can cause more damage to the system because it doesn't check whether the Master Boot Record of the computer has been infected or not. Therefore, if a similar virus has already infected the Master Boot Record, the virus will move the previous virus to the location the Master Boot Record was stored, making the recovery impossible.
Infected machines that aren't booted on March 6th won't be affected. The virus can stay dormant, waiting for years, until it's booted on that date.