With the help of the BableFish Translation http://babelfish.altavista.com/, it means “The body of the message was eliminated because a virus contained”
So the body of the email and not simply an attachment was infected and the only option was to delete the body.
For the most part tracking these is a waste of time as ther email from address is usually forged and since avast is dealing with them you are secure.
What is your email program?
When avast detected this did you not choose to delete the email completely and stop even the blank email even geting in to your inbox?
I use MailWasher Pro, although it is primarily for Spam it is also easy to deal with suspicious emails. There is a free version, but this only works with a single email account. The Pro version works with multiple accounts. There are other tools like this so you could delete them at source without having them downloaded.
Thanks for the advice - especially DavidR. I am reassured.
Avast did not ask me whether to block the email before it was delivered but it seems it did a good job of neutralising it anyway. I use outlook mostly, which collects mail from Yahoo (BT). Occasionally I intercept it via webmail before outlook gets it.