DavidR
2
This has been discussed a number of times.
The conclusion was some of the test samples, especially those in archive file attachments (which are inert unless activated) and you also have to open an attachment from an unknown email source (common sense applies, you wouldn’t do it, right?). avast also has defence in depth, you aren’t only relying on it being caught in a zip attachment, there is also the standard shield (on-access) provider when you try to save the zip to your HDD or try to open the zip file.