How Viruses Spread 
Knowing how viruses spread is a vital first step in preventing virus attacks. Viruses and other computer threats enter and leave computers through the same communication channels used to exchange information. These can be classified into three groups: the Internet, networks, and removable disks.
Internet
The Internet has become the most widely used form of sending and receiving information. Unfortunately, it has also become the fastest way to spread viruses and other computer threats. Viruses can be transmitted through e-mail attachments, network shares, chat programs, files downloaded from the web, and peer-to-peer file sharing.
Networks
A network is a group of interconnected computers that makes it possible to share information and resources. This makes working in groups easier but also offers a convenient channel for viruses to spread. If one computer on the network gets infected, others connecting to it will also become infected, causing a chain reaction that can paralyze the entire network.
Removable disks
Removable disks are storage devices on which data is stored. These devices include floppy disks, zip disks, CDs, DVDs, flash drives, and removable hard drives. If any of the files saved on a disk are infected, using the disk in another computer will spread the virus.