These programs secretly attach themselves to other programs. What makes them dangerous is that, before they do whatever damage they may be programmed for, they first copy themselves to additional program files. Thus, computer viruses infect and reproduce in a fashion somewhat analogous to biological viruses. The scope of the threat has changed significantly in the past few years, says Sal Viveros, director of McAfee Active Virus Defense at Network Associates Inc., a Santa Clara, Calif.-based vendor that develops programs to protect against malicious software. They were called boot-sector viruses because they booted off the floppy drive.
Viruses are still the biggest computer security problem.
That's only a fraction of the 50,000 known malicious software codes, Viveros says. Macro viruses, which are triggered by automated tasks within programs such as Microsoft Word, are today's biggest threat. Last month, they accounted for nine of the top 15 viruses, Viveros says. There are also some complex variations such as polymorphic and stealth viruses, which try to avoid detection by changing their internal structure, he says. And hostile Java applets are an emerging threat.