Antivirus software.
As viruses become a great problem for computers and cellar phones special programs for detecting and disposing of viruses were created. Such programs are called antivirus. The antivirus programs may be of five types: detectors, doctors, revisers, filters and immunisers. Detectors scan operating system and file in search of signature typical for one or another virus. Once they found it you will see the appropriate message on your display. The disadvantage of detectors consists in their ability to find only those viruses that are known for their creator. In case of infection computer by some unknown virus detector will find nothing and informs you that everything is okay, meanwhile virus will continue infection of your computer.
Antivirus programs that belong to doctor and immunizer types are able not only to detect the virus but also to heal infected file. Healing means a disposing of the virus's body of infected file, as soon as virus is disposed you may consider file as recovered. First of all doctors scan operating system and files and dispose virus and than they heal infected files. Among antivirus-doctors there are so called 'polydoctors' – antivirus programs that are able to detect a great variety of viruses. In condition of intensive development of virus technology doctors and immunizer live out of time rather fast, so these programs need regular updating of virus archive base.
Revisers are the most reliable antivirus programs as they fix source data about files', software, catalogues and disks' system sectors when computer is uninfected and than compares results of every scanning and source data and in case of mismatch inform you that your computer is infected. Revisers are programs that are able to detect invisible Stealth virus, there's no other antivirus able to perform this task without reserve floppy system loading.
Filters are residential antivirus programs aimed on detecting of performing suspicious activity of computer and its operating system. That activity may be presented as correction of com or exe add-ins files, changes at file's property, operating of DDR directed to absolute address, information saving on boot-sectors of hard disk or loading of residential program. As soon as any program starts performing one of these activities filter sends user a request about allowing or forbidding of certain activity. The main advantage of filters is the ability to detect virus on the first stage of its operating, before it infects the whole system.
Immunizers are also residential programs that prevent infection of files. Immunizers modify program of file so that virus will identify them as already infected, but operating of immunized file or program won't be troubled.