A computer virus is a type of malicious software with particular characteristics:
A computer virus requires a host program, and it needs to be initiated by an unsuspecting user. Triggering a virus can be as simple as opening a malicious email attachment (malspam), launching an infected program, or viewing an ad on a malicious site (adware). Once that happens, the virus tries to spread to other systems on the computer's network or in the user's list of contacts.
Computer viruses self-replicate. This process of self-replication can happen by modifying or completely replacing other files on the user's system. Either way, the resulting file must show the same behavior as the original virus. Notably, computer worms also self-replicate, but the difference between a virus and a worm is that viruses rely on human action for activation and need a host system to replicate.
A computer virus requires a host program, and it needs to be initiated by an unsuspecting user. Triggering a virus can be as simple as opening a malicious email attachment (malspam), launching an infected program, or viewing an ad on a malicious site (adware). Once that happens, the virus tries to spread to other systems on the computer's network or in the user's list of contacts.
Computer viruses self-replicate. This process of self-replication can happen by modifying or completely replacing other files on the user's system. Either way, the resulting file must show the same behavior as the original virus. Notably, computer worms also self-replicate, but the difference between a virus and a worm is that viruses rely on human action for activation and need a host system to replicate.