As originally conceived, the Internet is not just a means for moving messages between PCs. It was designed as a link between computer systems that allowed scientists to share machines. One researcher in Boston could, for example, run programs on a computer system in San Francisco. Commands to computer systems move across wires just as easily as words and images. To the computer and the Internet they are all data.
Much of the expense businesses put into connecting to the Internet involves undoing the work of the original Internet creators. The first thing they install is a firewall that blocks outsiders from taking control of the business's internal computer network. They must remain constantly vigilant that some creative soul doesn't discover yet another flaw in the security systems built into the Internet itself.
Can someone break into your PC through the Internet? It's certainly possible. Truth be known, however, rummaging through someone's PC is about as interesting as burrowing into his sock drawer. Moreover, the number of PCs out there makes it statistically unlikely any given errant James Bond will commandeer your PC, particularly when there's stuff much more interesting (and challenging to break into)-like the networks of multi-billion dollar companies, colleges, government agencies, and the military.
The one weakness to the above argument is that it assumes whoever would break into your PC uses a degree of intelligence. Even as a dull, uninteresting PC loaded with naught but a two-edition-old copy of Office can be the target of the computer terrorist. Generally, someone whose thinking process got stalled on issues of morality, the computer terrorist doesn't target you as much as the rest of the world that causes him so much frustration or boredom. His digital equivalent of a bomb is the computer virus.
A computer virus is program code added to your PC without your permission. The name, as a metaphor to human disease, is apt. As with a human virus, a virus cannot reproduce by itself-it takes command of your PC and uses its resources to duplicate itself. Computer viruses are contagious in that they can be passed along from one machine to another. And computer viruses vary in their effects, from deadly (wiping out the entire contents of your hard disk) to trivial (posting a message on your screen). But computer viruses are nothing more than digital code, and they are machine-specific. Neither you nor your toaster nor your PDA can catch a computer virus from your PC.
Most computer viruses latch onto your PC and lie in wait. When a specific event occurs-for example, a key date-they swing into action, performing whatever dreadful act their designers got a chuckle from. To continue infecting other PCs, they also clone themselves and copy themselves to whatever disks you use in your PC. In general, viruses add their code to another program in your PC. They can't do anything until the program they attach themselves to begins running. Virus writers like to attach their viruses to parts of the operating system so that the code will load every time you run your PC. Because anti-virus programs and operating systems now readily detect such viruses, the virus terrorists have developed other tactics. One of the latest is the macro-virus that runs as a macro to a program. In effect, the virus is written in a higher level language that escapes detection by the anti-virus software.
Viruses get into your PC because you let them. They come through any connection your PC has with the outside world including floppy disks and going online. Browsing web pages ordinarily won't put you at risk because HTTP doesn't pass along executable programs. Plug-ins may. Whenever you download a file, however, you run a risk of bringing a virus with it. Software and drivers that you download are the most likely carriers. Most webmasters do their best to assure that they don't pass along viruses. You should always be wary when you download a program from a less reputable site.
There is no such thing as a sub-band or sub-carrier virus that sneaks into your PC through a "sub-band" of your modem's transmissions. Even were it possible to fiddle with the operation of a modem and add a new, invisible modulation to it, the information encoded on it could never get to your PC. Every byte from an analog modem must go through the UART in the modem or serial port, then be read by your PC's microprocessor. The modem has no facility to link a sideband signal (even if there were such a thing) to that data stream.
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1. Your virus scanner may be useless
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