A Strange Letter

Before I left for class this morning, I noticed an envelope just inside my door. It was unmarked; no stamp, no sender, no address. The envelope was sealed, and when I opened it, the letter inside was even stranger. The envelope looked modern, as if it had never been used before, but the letter was old. I was able to scan the letter when I came home today;

If you can't read it, it says:
Mary,
Run, child, run as fast as you can.

It then is signed with only that strange mark, which reminds me of a bird. I don't know who sent it, but whoever did sent it directly to me, actually taking the time to hand deliver it. As for the message, all I can tell is it is a sort of warning, but I am unsure as to what. If I am the 'child', what is it I am running from? And..the hand writing. It seemed child-like, but also seems..different, somehow. I don't think a child wrote this. It is all very perplexing.