Introduction

Computer viruses are small software programs that are designed to spread from one computer to another and to interfere with computer operation.
A virus might corrupt or delete data on your computer, use your e-mail program to spread itself to other computers, or even erase everything on your hard disk.
Viruses are most easily spread by attachments in e-mail messages or instant messaging messages. That is why it is essential that you never open e-mail attachments unless you know who it's from and you are expecting it.
Types of Viruses:
There are a number of viruses and they may be classified on certain basis. There are a lot of bases on which computer viruses can be classified, such as origin, techniques, types of files they infect, where they hide, the kind of damage they cause, the type of operating system or platform they attack
Let us look some virus types.
A) Resident Viruses
B) Direct Action Viruses
C) Overwrite Viruses
D) Boot Virus
E) Macro Virus
F) Directory Virus
G) Polymorphic Virus
H) File Infectors
I) Companion Viruses
J) FAT Virus
K) Worms
L) Trojans or Trojan Horses
M) Logic Bombs