Viruses do not generate by chance. the programmer of a virus, known as a virus author, intentionally writes a virus program. writing a virus program usually requires significant programming skills. the virus author ensures the virus can itself, conceal itself, monitor for certain events, and then deliver its payload. "

The answer is 'replicate'. Computer viruses, much like normal viruses, exist only to replicate themselves. The illnesses that people feel when they catch a virus, are to facilitate the spread (replication) of the virus, such as a runny nose and sneezing. Viruses are really just packages of genetic material. Richard Dawkins, in his book 'The Selfish Gene', proposes an alternative view of life on Earth: Organisms, including ourselves, are merely vehicles for their DNA, or a way to ensure their DNA is replicated. 

Because some people pollute water ans waste it