Viruses are programs which are deliberately written to damage data.
They become resident in the hard drive until a trigger event when they will be executed. It attacked computer systems world-wide, turning data on hard disks into nonsense. Some viruses can be harmless yet irritating pranks, whilst others cause disruption, loss of data, and can even crash computer systems. The most dangerous aspect of viruses is their ability to copy themselves from files and disks. In order to prevent viruses it is necessary to implement regular virus checks with anti-virus software.