Most viruses are harmful which cause random damage to data files and systems.
Some, however, may be present for years with no noticeable effect. They occupy disk space, main memory and CPU processing time. In most cases, all write-enabled floppies used on that PC from then on will become infected. Stealth viruses Viruses that go to some length to conceal their presence from programs which might notice. These types of viruses are usually easy to detect.