A computer can become infected by files downloaded over a network, or by the installation of new software or floppy disks that are infected with viruses. Some viruses are only pranks, and perform harmless actions like displaying a screen with a joke message on it. Others can destroy files or wipe out a hard drive. To avoid damage from viruses, write-protect the boot disk and other important disks, check new software or disks for viruses, and have virus protection software installed on the computer at all times. Disinfectant programs must be updated periodically because new virus get into circulation over time.
Viruses can be benign.
Vigilance is your best defense. My home computer has been infected by one virus, which in turn infected about half of all of the floppy disks I owned at the time. Use the best protection you can afford, whether that means freeware or expensive commercial protection. It needs protection too. Spread knowledge, not fear.