Types of Infection

There are many ways for a virus to enter a computer system:

Floppy Disks

Email

Network

Software

Internet 

Floppy Disks

Floppy disks are still probably the number one way that viruses are spread. Floppies are a major source of virus infection for two main reasons: first, because they are used to carry files from PC to PC, and second, because they are the only way that boot sector infector viruses can be transmitted. Infected  floppies with file infector virus can transmit to other PCs when the user copies an infected program from the floppy to the hard disk of the destination PC. When the copy is executed, the virus will be loaded into memory and then will be able to infect the hard disk and other programs on it. Other removable media such as Zip disks can also  be used to pass on infected files from one computer to another. The more important way that floppies are responsible for virus transmission is that they are the primary way of spreading for boot sector infector viruses. The reason is simple: most of these viruses are designed to infect boot sectors, and they use the boot process to get themselves into memory. Floppies are the ideal way of transmitting these viruses because each one has a boot sector, and most systems try to boot them. A common way that boot viruses spread is to infect a non-bootable disk. The user takes it to a PC and puts it in the drive for whatever reason. He turns the PC off. Later, he turns it on, forgetting to eject the floppy. The system attempts to boot the floppy. It fails, but the virus is loaded into memory, and infects the hard disk's boot sector. At this point the damage is done, even if the floppy is later removed.

Email

Email virus infection is  the worst things that can possibly happen to the  computer. This is also one of the most popular methods of spreading viruses. The  common way of spreading comes in the form of an attachment to the message. The attachment is normally used for emailing documents, images, and so on. However, it is also possible for attachments to contain programs which get run when the attachment is opened. The user can't get a virus just by reading a plain-text E-mail message or Usenet post. What the user have to watch out for are encoded messages containing embedded executable code, such as JavaScript in an HTML message or messages that include an executable file attachment, such as an encoded program file or a Word document containing macros. Usually, viruses, such as worms, spread themselves by sending infected messages to all of the email addresses contained on the infected computer.

Network

Virus infection over PC networks are becoming an increasingly difficult problem. A network provides the ability for users to share files, and where files are shared, viruses can be shared. With stand-alone computers, viruses can only spread via floppy disks, but on a network some viruses can spread without a disk ever being used. First, The virus enters a PC on a diagnostic disk or salesman's demo. It remains in memory after the visit. When the first program is run, it  becomes infected. After that,  every program that is run  also becomes infected. On reboot, the virus  is cleared from memory, but it is re-installed when an infected program is run.  Eventually a program that is run from the file server becomes infected. Then, another user runs the infected program from the file server. The virus installs itself on this PC, and infects any programs run from there, including those on other devices. Thereafter, any user running the infected program from the file server will spread the infection, so the virus spreads more quickly than  the  stand-alone computers.

Software

Many people don't realize that  viruses are sometimes spread through software distribution. While usually this happens with pirated software, low-budget freeware or shareware products. Pirated software is a common source of viruses. People who install illegal programs on their computer are taking an enormous risk of virus infection. Pirated software  may carry a virus from the machine that it was copied from, or copied to and then recopied with the virus included. Games are probably the most commonly pirated software and they are more likely to move between PC users at a far greater speed than the other pirated software. As a result, they are more likely to pick up a parasitic virus along their journey. Shareware is  another possible virus carrier. Most authors send the latest version of a specified package once payment has been received, but users will sometimes try the original version obtained form a friend of a friend of a friend. Once that person receives the latest version, the computer may have been infected numerous times with any viruses carried by the original software that may have been picked up on its way. It is possibility  that viruses can be found even on commercial, shrink-wrapped software. There is no guarantee that a package has not been previously opened and the software had been used and infected, and then repackaged prior to final sale.

Internet

Virus infection via files downloaded from the Internet is probably the fastest growing area of infection. More and more companies are making software available over the Net. Virus may infect the system through WWW, if the user downloads an insecure file from Internet. For instance, if the user download a program from the Internet and it is infected with a virus, the virus will attack your computer when you activate the program. Also, computer can get infected by viewing infected web pages. Although HTML cannot carry a virus, web pages with HTML code can contain code that runs applications automatically, in turn making the code capable of launching an infected operation.