Viruses : a better definition

Viruses are another form of mobile code. Only unlike the examples above, viruses are not invited in at all. The difference between a virus and ordinary mobile code is that viruses are written to reproduce themselves. When a virus arrives, either via a Web page, an e-mail attachment, or some other way, it usually starts out be infecting executable programs on the disk. When one of these programs is run, control is transferred to the virus, which usually tries to spread itself to other machines, for e.g., by e-mailing copies of itself to everyone in the disk, so when the machine is booted, the virus gets to run. Viruses have become a huge problem on the Internet and have caused billions of dollars worth of damage. There is no obvious solution. Perhaps a whole new generation of operating systems based on secure microkernel and tight compartmentalization of users, processes, and resources might help.