The attachment could be one of several classes of viruses. Two common types are Trojan horses and worms. Trojan horses allow a hacker to break into a victim's computer when it's on the Internet and steal or alter files. Worms can destroy files and e-mail themselves to everyone in the victim's address book. Viruses usually do no physical harm to computer hardware, but one virus is known to have the ability to delete the embedded software on a computer's motherboard, making it useless, Arthur said.
Polymorphic viruses offer an extra challenge by disguising themselves to elude antivirus software.
A first line of defense is to use caution when dealing with attachments, especially from strangers, but also from friends. Worms often are unknowingly spread by friends or business associates. Some Internet service providers, companies and schools now filter e-mail messages with attachments before infected messages get delivered. At Drake University in Des Moines, the recipient of an infected e-mail will still get the text of the message, accompanied by a statement saying that a virus was found and deleted. The school catches numerous e-mail viruses every day, said Robert Wood, associate professor of information systems at Drake University.