I had no reason to expect a confidential document from Mr. Ullyatt. He was a trusted source, to move back to your virus analogy. I quickly opened the e-mail, saw it was from him, and didn't go back to it until many hours later in the day. At the end of the day, when I was printing stuff to clear my desk for the day, I quickly printed it, along with a number of other documents. I quickly did a scan and threw it in a drawer.
It never occurred to me that it would be an inappropriate document, that he would send me an inappropriate document, or that he would have an inappropriate document. Therefore, I reacted the way I did.