A virus is not a separate program but rather a program part included (appended at the end) in some other program. At the beginning there is a pointer to the virus appendix. If the infected program is running, it is loaded into memory. The virus remains in memory until you shut down your computer and may infect any other program that you run after. And so on. Viruses spread over networks or by means of diskettes.
A macro virus uses macro programming languages (for instance of Microsoft Word, Excel, ...). These viruses may even be transmitted by documents, since the macro definitions are written on the documents too.
A stealth virus tries to protect itself from virus scan programs. If the scan program tries to scan, the virus recognizes this and gives the original file to the scanner.
Some viruses try to hide their program status by using double extensions like "File.jpg.exe". Depending on your Windows setting, only "File.jpg" might show. Even the file name "trkgif.exe" is meant to mislead, I suppose.
You may get a virus by
Antivirus software is supposed to detect and destroy viruses. But you have to update this software regularly.
Firewalls are also important.
Erich Prisner, August 2003