Difference between virus and the program of computer

1. Virus can get important information, example: virus can know the operation system from computer.
2. Virus have ability to investigate program and file, example: virus can turn off the antivirus program.
3. Virus can multiply itself without command from user, but it must be executed at first.
4. Virus can deceive user. Example virus with icon Word, or icon image.
5. Virus can hide itself and data which are made.

We must be carefull to execute the program from outside computer. Flash disk and flopydisk are media to disseminate virus or worm. The extension of virus is usual .exe or .com.