It is just now, that the cause has been determined. It is a new virus that is being transmitted through ticks, that is just hitting the United States.
Because the disease is viral, no antibiotic can treat it (as is the case with lyme disease), and as such is posing a terrible threat.
According to the infectious disease experts studying this, nothing like this has ever been seen in the Western hemisphere. Its genome is similar to diseases seen in Eastern Europe, Africa and Asia.
Because the man who died lived in Bourbon County, Kansas they have dubbed this illness the Bourbon virus which the infectious disease authorities believe is being carried by ticks.
It is being studied intensively to determine what in the genome caused the anorexia, severe muscle pain and extremely high fever that was untreatable and ultimately killed this man.