Viruses can infect a number of different portions of the computer's operating and file system. These include:

• System Sector Viruses: These infect control information on the disk itself.
• File Viruses: These infect program (COM and EXE) files.
• Macro Viruses: These infect files you might think of as data files. But, because they contain macro programs they can be infected.
• Companion Viruses: A special type that adds files that run first to your disk.
• Directory (Cluster) Viruses: A special type that infects through the disk directory.
• Batch File Viruses: These use text batch files to infect.
• Source Code Viruses: These add code to actual program source code.
• Visual Basic Files: These worms use the Visual Basic language to control the computer and perform tasks.
• Screensavers: These are not just pictures but executable code.
• Vulnerabilities: Problems in operating system or program code can be exploited.

That's a summary, now see how each of these might work by continuing on in the tutorial.

Types of Viruses
Types of Viruses	System Sectors