When the user opens an infected file, the virus code runs often without the user being aware of it. The virus depends on users to spread because it has no mechanism by which it can move from one computer to another. The virus must be passed on to other users through infected e-mail attachments, programs on diskettes, or shared files. Another popular payload involves spawning network processes that in turn start other processes. As a result, the overburdened network fails.
Some viruses are easy to locate and eradicate because they make exact copies of themselves.
An antivirus scanner can identify the program s signature and warn the user. An encrypted virus makes things a bit more complex. By scrambling the signature, encryption makes it impossible for antivirus software to spot it. Instead, the scanner must look for a decryption routine that the virus uses. These viruses are fairly easy to detect.