Computer viruses and worms are an increasing problem throughout the world. By some estimates 2003 was the worst year yet: Viruses halted or hindered operations at numerous businesses and other organizations, disrupted cash-dispensing machines, delayed airline flights, and even affected emergency call centers. The Sobig virus alone is said to have caused more than $30 billion in damage (1). And most experts agree that the damage could easily have been much worse. For example, Stamford et al. de-scrihe a worm that could infect the entire Internet in about 30 s (2). A worm of this scale and speed could bring the entire network to a halt, or worse. The term virus refers to malicious software that requires help from computer users to spread to other computers. E-mail viruses, for instance, require someone to read an e-mail message or open an attached file in order to spread. The term worm refers to infections that spread without user intervention. Because they spread unaided, worms can often spread much faster than viruses. Computer infections such as viruses and worms spread over networks of contacts between computers, with different types of networks being exploited by different types of infections. The structure of contact networks affects the rate and extent of spreading of computer infections, just as it does for human diseases (3- 7); understanding this structure is thus a key element in the control of infection.
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