What is Virus?

The term virus means a simple living thing,smaller than bacteria, which causes infectious disease. But now-a -days  this very term is used in computing too. Among them most dangerous are the  viruses.There are over a thousand viruses. Recently, we have heard of the computer virus affecting a good number of  computers in our land which has been termed as"Chernobil Virus," But it is actually beyond the conception of a lay man what actually happened and how did it happen . Only the experts  alone can give light on this point.