A virus infection need not be accidental. Simply insert a floppy and issue a command and an entire network can be brought down for days. The current trend in large corporations is to downsize computer centers, usually by migrating to PC networks. With most PC networks there is far less physical control over the security of data. What was a mandated security procedure becomes an operational suggestion, at best implemented whenever employees remember to follow procedures.
Security is no longer keys and locks, but a series of logon and password combinations.
As a result, just as many companies have had random purse-snatching or other acts of violence within their offices, terrorists can just as easily damage data on a PC or network. Security is becoming much more than simply avoiding data theft. Security must also provide for data protection. Intentional data damage is far easier to arrange and harder to detect. It might take days, or even weeks, to determine that the data is actually wrong.