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Attack Phase

Viruses need time to infect. Not all viruses attack, but all use system resources and often have bugs.

Viruses usually delay revealing their presence by launching their attack only after they have had ample opportunity to spread. This means the attack may be delayed for days, weeks, months, or even years after the initial infection.

The attack phase is optional, many viruses simply reproduce and have no trigger for an attack phase. Does this mean that these are "good" viruses? No! Anything that writes itself to your disk without your permission is stealing storage and CPU cycles. (See also the discussion on good viruses.) This is made worse since viruses that "just infect", with no attack phase, often damage the programs or disks they infect. This is not an intentional act of the virus, but simply a result of the fact that many viruses contain extremely poor quality code.

An an example, one of the most common viruses, Stoned, is not intentionally harmful. Unfortunately, the author did not anticipate the use of anything other than 360K floppy disks. The virus will try to hide its own code in an area of 1.2MB diskettes that results in corruption of the entire diskette.

Summary:

Viruses need time to establish a beachhead so even if they activate, they often will wait before doing so.

Not all viruses activate, but all viruses steal system resources and often have bugs that might do destructive 
things.