What is computer virus how can we protect our computer from it

Virus:
A computer virus is a software program which performs a malicious function that infects the other programs or damages the disk. This virus replicates itself within a computer system.

Function of viruses:

1. It infects the other programs. 
2. It damages the disk it destroy the data of the disk. 
3. Sometimes it makes the PC unusable. 
4. It increases he file size of the file on the disks, but not effect its functioning. 

Protection From Viruses: 

1. Never boot you system, it ensure that the floppy used for booting a system is free from virus. 
2. A system booting disk must be kept write protected. 
3. Avoid using .com, .sys or exe files not know to you. 
4. Avoid from the users of pirated softwares. 
5. Always use only virus free programs and disks.