What kind of files can spread viruses?

Viruses have the potential to infect any type of executable code, not just 
the files that are commonly called 'program files'. For example, some 
viruses infect executable code in the boot sector of floppy disks or in 
system areas of hard drives. Another type of virus, known as a 'macro' 
virus, can infect word processing and spreadsheet documents that use 
macros. And it's possible for HTML documents containing JavaScript or other 
types of executable code to spread viruses or other malicious code. 

Since virus code must be executed to have any effect, files that the 
computer treats as pure data are safe. This includes graphics and sound 
files such as .gif, .jpg, .mp3, .wav, etc., as well as plain text in .txt 
files. For example, just viewing picture files won't infect your computer 
with a virus. The virus code has to be in a form, such as an .exe program 
file or a Word .doc file, that the computer will actually try to execute.

Articles from COMPUTER VIRUS FAQ