Now that I've said that, let me backtrack. Viruses are programs, and as such they can be transported on the back of an email message, but in no way will opening email activate an infected program. Be sure that you scan any programs that are sent to you on an email message before you run it. Even if it comes from someone you know and trust, they could be transporting a virus unawares. Viruses fall into various categories depending on how they infect your computer.
Some virus programs take on two or more different characteristics within the same virus.
For instance, there are several viruses that infect both your boot sector and use stealth techniques to hide from detection. These infect both systemic areas such as boot blocks and executable files. These viruses are opportunistic, infecting the available files at random using a combination of different techniques. An example would be Tequila . It is a combination of a Stealth, File, and Boot viruses all in one virus.