Danny Orlis and the Time of Testing

Kay and Danny are faithfully serving God as missionaries in Guatemala. While Danny is out clearing ground for an airplane landing strip, Kay is asked to make an urgent medical visit upriver in the jungle. On the way back to the mission station, Kay shows signs of heatstroke, and Danny and Kay must return to Minnesota for Kay's recovery. While in Minnesota, Kay receives bad news from the doctor that doesn't make sense. Even though she feels fine, she's told that she shouldn't return to Guatemala. Why would God allow this to happen when they are so needed in Guatemala?