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Write a note on Computer virus?

Viruses are collection of coded instructions which are self replicating. When a virus attaches
 itself to another file it infects it. They are normally inactive until infected program is run.
 They are broadly classified into three categories.
 a. Boot infectors
 b. System infectors
 c. Executable program infectors
 Boot infectors: Create bad sectors. They remain in the memory until the system is shutdown.
 System infectors It infects the hard disk or: bootable floppies which may contain system files.
 Executable program infectors: These are dangerous and devastating. They spread to
 almost any executable program attaching themselves to programming files.