To fully understand the concept odlf a virus or worm first recall the concept of a computer program what can a program do and how a program is executed a program is a sequence of instruction given to a computer these instruction are for some sort of processing by processing we mean performing arithmetic and logical operations on the data in the computer memory manipulating data is storing and arranging two data item to see if they are equal or not and transmitting data in sending data form one computer to some other computer on a network or via internet. A virus is a program that attaches itself with other executable files by modifying them so that the virus program is also loaded and executed with execution of these program a virus usually perform destructive operating by deleting or modify unknowingly data stored on the storage devices attached to the computer.
It is important to know exactly how a virus reaches from one computer to another as the virus attach itself with files present on a system so the only way virus can reach from one computer to another is when some data is exchanged between these computer either through floppy disk or the network.
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