Complex sentence: In October of 1989, when an unusually high number of their monkeys began to die, their veterinarian decided to send some samples to Fort Detrick (USAMRIID) for study.
Split into simpler sentences: In October of 1989, an unusually high number of their monkeys began to die. Their veterinarian decided to send some samples to Fort Detrick (USAMRIID) for study.