... But these 'juvenile' hackers eventually get caught, and the reason they get caught is due to the fact that they're not professionals. The second type of hackers are however the professionals, they're the ex-KGB agents, the ex-CIA agents, the person from German intelligence, or the Israeli intelligence and they're not going to get caught. A computer virus is a program (a block of executable code) which attaches itself to, overwrites, or otherwise replaces another program in order to reproduce itself without the knowledge of the user. A vast array of viruses are comparatively harmless and may be present for years with no noticeable effect. Some, however, may cause random damage to data files (sometimes insidiously, over a long period) or attempt to destroy files and disks. Others cause unintended damage. Even benign viruses (apparently non-destructive viruses) cause significant damage by occupying disk space and/or main memory, by using up CPU ...

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