Weird co-worker.

My husband works in an office environment with several co-workers. For the most part everyone he works with is great. There is however, this one guy, who is just standoffish. He refuses to speak to anyone unless absolutely necessary and won't participate in group activities. In general he seems like he just needs a herbal colon cleanse. I have to admit some of the things I hear about him worry me a bit. In today's world this is nothing to joke about. I am constantly reminding my husband to not interact with this guy on anything but a professional level because you just don't know what the guy is thinking. Despite working right next to people the guy will send out and email rather than speaking when he needs something. I understand shyness but there is just something more to this that I can't put my finger on.