A computer virus is a piece of written code that copies itself, spreads itself around, and tells a computer what to do.
It might, for example, infect all the IBM-compatible computers it can, and on the next Friday the 13th, find all the decimal points in the data where it is and move them one digit to the right. Or it might simply destroy all the data and write “Gotcha” on the screen.
Human viruses are spread by all sorts of contact. Anyone who works in an open office knows how fast a cold can spread, with everyone sneezing on each other, sharing telephones, shaking hands. Computers obviously don’t sneeze. But they have plenty of chances to come in contact with each other, and every contact is a chance to transmit a computer virus.
Virus codes can be added to the lines of written code that make up computer programs, hidden among thousands of lines of commands that tell the computer what to do, and when.
A computer hacker, one of those highly capable computer programmers who works day and night at his keyboard, could write a virus into a “bootleg” program before giving a copy of the disk to a colleague. Once the disk is inserted into a healthy computer, the virus copies itself onto other program files on the main computer and even into the computer’s operating system, its brain.
The next time anyone inserts a disk into the infected computer to play a computer game, the new disk will be infected too. Or if the computer sends some program by modem, the virus goes along. And when the computer taps into a network, the virus sends itself, too. Traveling by network, a virus can infect hundreds of thousands of computers in just a few hours. Most viruses are started by one person who’s up to no good, but very few are spread intentionally.
In 1988, a Cornell University computer science graduate student created a virus and released it into two networks that scientists use for exchanging information.
The student meant only to show off his skills by gaining access to hundreds of research facilities’ computers. But the virus went wild, replicating itself faster than the student ever intended and crippling computers all over the United States. The Computer Virus Industry Association estimated that $98 million was spent to clean up the damages left in its path.
Expert hackers have proven themselves capable of finding access to corporate and federal government computers and leaving virus code behind. Sometimes this is done through “trapdoors,” the secret ways into a system that avoid the usual security measures like passwords.
In 1988 the Chaos Computer Club of West Germany gained access to NASA’s computer and maintained access for five months before being discovered. Its members claim to have left behind viruses that will activate in the future, although NASA’s own computer experts deny this.
Virus guards, codes written by the good guys, have helped curb the spread of viruses. They can detect typical virus language (and some can detect particular, known viruses) before the infected program runs on a computer.