A serious illness, called Rocky Mountain fever, is caused by a tiny organism called a rickettsia. It invades the cells of the body and produces severe fever. The disease is carried by woodchucks, but wood ticks, living on these animals, can carry it to humans. The disease was first discovered in Montana, about 100 years ago, when it killed three-quarters of those who got it. Since then, it has been found in other parts of North and South America, but not in such a deadly form.