Some of the most dangerous computer viruses have been spread by E-mail attachments, which are activated only upon opening.

A. In theory, there are no official size limits across the Internet pertaining to E-mail attachments.

For this reason, users are routinely advised never to open an E-mail attachment from a stranger.

Opening E-mail attachments from people one does not know and trust is perhaps the most likely and dangerous way to admit and propagate a virus.

Then they can easily be archived or sent as E-mail attachments.

A number of companies offer novel ways to publish digital pictures on the Web or send them to Grandma as an E-mail attachment.

The attacking program shows up as an E-mail attachment.

Q. I use America Online and cannot open many E-mail attachments.

I'll send this as an E-mail attachment.

The virus, which began to cause havoc on Friday, is like a chain letter spread in an E-mail attachment listing several pornographic Web sites.