Now- a-days, most of the viruses spread due to receiving unknown e-mail messages that contains viruses. When a user opens such an infected message, the virus is also loaded into the computer memory. In this way, many other program files loaded into the memory are infected. This virus is also transferred to other computers when e-mail messages are sent from the infected computer to them. Due to these viruses, some time an auto generated email message from your email ID sends to your friends from your side with different virus messages. Never open unknown and attractive subject line email messages, always scan first even if you know the sender of the message.
Another way of spreading virus is by using Internet and other networks. For example, when you download infected executables files or data files from the Internet or from a shared disk on the network, viruses are transferred to your computer. It must be noted that many software are available on the Internet with free of cost. Most of that software contains viruses, for example free PHP or ASP scripts are the main source of virus, always get these scripts from reliable sources. In an LAN, if a computer contains a virus, then all the computers on the network may be infected with that virus.
One important means of exchanging data is through the use of removable media like, CDs, MP3 player, ipod and flash devices. So, when you copy the data from one computer to another by using a removable media, the viruses are also transferred.
The virus can also infect your computer by using pirated software. The software, which is installed into your computer without license is referred to as pirated software. Some companies may intentionally attach some virus programs into their software. This program will only activate when it does not find some special files like license files on your computer.