How does a computer virus harm the computer?

Virus can to much harmful for our computer like :-

1) Damage files
2) Damage programs
3) Delete files
4) Corrupt the operating system

Also,there are 4 types of viruses
1) General Viruses :- General computer viruses are packages of software that hide alongside other programs that seem valid. In most cases the virus is so small, and so well hidden, that by the time a user realizes it has "piggybacked" with his downloaded program, the virus has already started to cause its damage. Most general viruses act like human viruses. They do this by entering your system, and then taking advantage of it by using its components to duplicate itself and spread.

2)Trojans :- A trojan virus, sometimes referred to as a trojan horse, is named after its historical namesake. Just like the trojan horse in world history, a trojan disguises itself as something that it's not, but does not make itself known until it has made it onto your computer. Trojans can masquerade as games, programs or documents that you download online. Once you open the executable file, the trojan installs instead of the program and infects your computer. The trojan can then cause its damage, sometimes even erasing your hard disk. Unlike a general virus, a trojan is unable to duplicate itself automatically.

3) Worms :- A worm is an infection that exploits network connections. When a worm downloads itself onto your computer it uses network connections to other computers, as well as security holes to duplicate itself and spread. A worm uses one computer on a network system to scan all the other computers it is connected to for security holes. Once the worm finds a hole on another computer on the network, it copy and pastes itself into that computer by exploiting the security hole. It continues replicating until it runs out of computers to infect.

4)Email Viruses :- An email virus spreads as an attachment to an email. Sometimes this attachment can be seen, and other times it can't. Some email viruses can load onto your computer simply by opening an email into the preview frame. Once the virus has been opened, it automatically downloads its attachment to your computer. Some email viruses can also hijack your computer and send out copies of itself to everyone on your contacts list.