Help with Infection Blocked message Please

I am new to avast and a novice when it comes to Avast and virus etc.
i keep receiving a message saying Infection Blocked Avast mail shield has blocked a threat.
Infection: MW97:downloader-H(trj)
Subject: termination of deliveries.
From: “hydrotrcn”
File: INFO.doc

I don’t know what I need to do or how to stop this message from appearing.

Could anyone help me please.

I am on an IMAC running OSX 10.9.4

Thanks

Go to your mailbox and remove the infected mail, that’s all You have to do.

Hi

Thanks for your help, I have checked my mailbox and cannot find anything that relates to the blocked infection.

I have checked inbox, spam and deleted items.

Message appeared again 20+ times

Are there any related messages in the system log? What mail client do you use?
Can you find the mail in the gmail web interface?