I too thought that coyotes would not attack a person unless sick
When I was hiking this summer up in the mountains in Wyoming I heard a coyote yelping VERY close to the trail.
I admit it did scare as I took cover behind a stand of evergreens and picked up a large rock....I did have my dog with me and I wonder if she could have been the potential target?
I guess my bear pepper spray is not just for bears...
Here is a pretty good article that is from National Geographic that might be of use.
http://news.nationalgeographic.com/news/2005/06/0607_050607_coyotes_2.html