A virus is a program which replicate by copying itself, either exactly, or in a modified form, into another piece of executable code. Viruses can use many types of hosts, some of the most common are:

• Executable files (such as the programs on your computer)
• Boot sectors (the parts of code that tell your computer where to find the instructions it uses to 'boot' or turn on)
• Scripting files (such as Windows Scripting, or Visual Basic script)
• Macros within documents (this is much less common now, as macros in, for instance Microsoft Word, will not execute by default)

When a virus inserts itself into other executable code, this ensures it is run when that other code is run, and the virus spreads by searching for other 'clean' hosts every time it is run.

Some viruses overwrite the original files, effectively destroying them, but many simply insert themselves in a way that they become part of the host program, so that both survive.

Depending on the way they are coded, viruses can spread across many files in the system, across networks via file shares, in documents, and in the boot sectors of disks.

Although some viruses are spread by email, this does not make them viruses, and in-fact, most of the things that spread in email are actually worm.

To be a virus, the code simply has to replicate, it does not need to do a lot of damage, or even spread very widely (See payload).