A few years ago, news began circulating about a computer virus named Michelangelo thatwas set to "ignite" on March 6, the birthday of the famous Italian artist. The virus attacheditself to the computer's operating system boot sector

On the magical date, the virus would release itself, destroying all of the computer's data. When March 6 arrived, the virus did minimal damage. Preventive techniques limited the damage to isolated personal and business computers. Though the excitement surrounding the virus was largely illusory, Michelangelo helped the computer-using public realize its systems' vulnerability to outside attack.
What is a computer virus? Cite at least three reasons why no system is completely safe from a computer virus.

A computer virus is a segment of executable code that attaches itself to an application program or some other executable component. When the hidden program is triggered, it makes unauthorized alterations in the way a system operates.
There are a number of reasons why no one is completely safe from a virus:
• Viruses are contagious and are easily spread from one system to another. A virus spreads when users share programs or data files, download data from the Internet, or when they access and use programs from external sources such as suppliers of free software.
• Viruses can spread very quickly. In a network environment, a virus can spread to thousands of systems in a relatively short period. When the virus is confined to a single machine or to a small network, it will soon run out of computers to infect.
• Many viruses lie dormant for extended periods without doing any specific damage except propagating itself. The hidden program leaves no external signs of infection while it is reproducing itself.
• Many computer viruses have long lives because they can create copies of themselves faster than the virus can be destroyed.