Viruses can affect any files; however, usually attack .com, .exe, .sys, .bin, .pif or any data files - Viruses have the capability of infecting any file; however, will generally infect executable files or data files, such as word or excel documents that are opened frequently and allow the virus to try infecting other files more often.
Increase the files size - When infecting files, virtues will generally increase the size of the file; however, with more sophisticated viruses these changes can be hidden.
It can delete files as the file is run - Because most files are loaded into memory, once the program is in memory the virus can delete the file used to execute the virus.
It can corrupt files randomly - Some destructive viruses are not designed to destroy random data but instead randomly delete or corrupt files.
It can cause write protect errors when executing .exe files from a write protected disk - Viruses may need to write themselves to files that are executed; because of this, if a diskette is write protected, you may receive a write protection error.
It can convert .exe files to .com files - Viruses may use a separate file to run the program and rename the original file to another extension so the exe is run before the com.
It can reboot the computer when executed - Numerous computer viruses have been designed to cause a computer to reboot, freeze, or perform other tasks not normally exhibited by the computer.