1. Portable Storage Media
The storage media that is used to transfer files from one computer to another, is one of the biggest reason to spread virus in different computers. For example, in near past, people used foloppy disks to copy files from one computer to another. If the files being copied are infected with virus then virus will also be copied on the second computer. Similarly, computer virus can also be transfeerred from one computer to another, if we use USB flash drives, Zip Disks, Portable Hard Disks or Optical disks, different types of Storage Cards etc. for file copy process.
Nowadays, most viruses are spread through the internet. The internet has made viruses spread in a much quicker way. Computer viruses can attache to different files, making them infected. Therefore, if your PC is infected with a virus then sharing of infected files with other people, will transfer the virus, too. There are two main sources of virus on internet as follows:
a) E-Mail Attachements
Computer Virus may be spread through Email. Normally, an Email contains some files with it called Email Attachements. These attached files may be infected with computer viruses. So, if some one opens such Email and downloads/ opens the infected files, the computer may be infected with virus. Some Email viruses may send virus automatically to all email addresses saved in the email address book.
b) Insecure Websites
There are thousands of rogue websites on the web that can infect computer with viruses. When you visit a rogue website, it will offer some material to download like a game, a screen saver or a software. The computer virus is transferred to the computer when the material is downloaded. This virus file when openned will be active and may destroy data, files or software.
3. Computer Networks
A Computer Virus can spread if the user connects to a infected computer network. Let a NetCafe is running a Local Area Network to share Interenet Connection and files on Disk Storage. Now if a user inserts an infected flash USB drive in a computer system, the virus may be transferred to that comp[uter. Since the infected computer is linked with other computers in network, this virus may transfer to all computers while copying infected files.
4. Infected Pirated Software
An illegal copy of a software is called Pirated Software. Pirated software is not a licensed software. There are millions of free software available on CDs and Internet that are actually Pirated software. These infected pirated software may transfer virus in the computer system if installed. There are some companies that intentionally copy virus in pirated software. So that if people buy and install unlicensed software, the virus will be activated and may harm the computer system.
Example of Infected Pirated Software Virus:
On 26th of April, 1999 a virus Called CIH was activated in computer systems using Pirated Version of Windows 95 and Windows 98. The virus was already included in Pirated software CDs. CIH used the time bomb logic to be activated automatically when the computer system’s date is 26th April.
The virus was created by Chen Ing-hau, (hence the name CIH Virus)who was a student at Tatung University in Taiwan. 60 million computers were believed to be infected by the virus internationally, resulting in an estimated $1 billion US dollars in commercial damages.
CIH’s dual payload was delivered for the first time on April 26, 1999, with most of the damage occurring in Asia. CIH filled the first 1024 KB of the host’s boot drive with zeros and then attacked certain types of BIOS. Both of these payloads served to render the host computer inoperable, and for most ordinary users the virus essentially destroyed the PC. Technically, however, it was possible to replace the BIOS chip, and methods for recovering hard disk data emerged later.