IN THE WAR AGAINST DISEASES, THEY ARE THE SPECIAL FORCES.They always keep a bag packed. They seldom have more than twenty-four hours' notice before they are dispatched. The phone calls that tell them to head to the airport, sometimes in the middle of the night, may give them no more information than the country they are traveling to and the epidemic they will tackle when they get there.The universal human instinct is to run from an outbreak of disease....