5.Computer virus

Small but insidious piece of programming-code that attacks computer and network systems through 'contaminated' (infected) data files, introduced into a system via disks or internet. As a digital equivalent of biological microorganisms, it attaches itself to the target computer's operating system or other programs, and automatically replicates itself to spread to other computers or networks. Invented in 1960s as a prank, viruses come in thousands of types and versions with new ones being invented every day, each requiring a different cure (see antivirus and vaccine). While a few viruses are harmless diversions, most are malicious and cause widespread and severe damage and may bring down entire communication-networks or websites.
Some are immediately active, others remain latent for weeks or months, or work slowly to avoid detection and cause destruction over long periods. Propagation of computer viruses is a serious crime in many countries. See also Trojan horse, and worm.