Common word is VIRUS

Virus is very harmful in our life.various kind of virus has in this world.people hates it.nobody like it.among them Computer Virus is one of them.All over the world know it.99% people it hates but 1% people it creates.it is very harmful in our life.it hamper our pc or laptop etc.We don't like it.whatever,it removes our important files.it crash our official important files.
Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer.
alware includes computer viruses, computer worms, Trojan Horses most Rootkits,Spyware dishonest adware and other malicious and unwanted software, including true viruses. Viruses are sometimes confused with worms and Trojan horses, which are technically different.Some viruses do nothing beyond reproducing themselves.

The first academic work on the theory of computer viruses as done in 1949 by John Von Neumann.But that time computer virus was not used.
The Creeper virus was first detected on ARPANET, the forerunner of the Internet, in the early 1970 Creeper was an experimental self-replicating program written by Bob Thomas an BBN Technologies in 1971.

many viruses attach themselves to executable files that may be part of legitimate programs. If a user attempts to launch an infected program, the virus' code may be executed simultaneously. Viruses can be divided into two types based on their behavior when they are executed. Nonresident viruses immediately search for other hosts that can be infected.
Resident viruses contain a replication module that is similar to the one that is employed by nonresident viruses.
 Resident viruses are sometimes subdivided into a category of fast infectors and a category of slow infectors.The slow infector approach, however, does not seem very successful.

when it is highly increased that time hard disk crashes.
A true virus can spread from one computer to another when its host is taken to the target computer; for instance because a user sent it over a network or the INTERNET, or carried it on a removable medium such as a floopy,CD or DVD or USB Drive.

Now-a-days various anti virus has in this world for virus proptect.so,we should  it uses for protect.