Answer:
Follow the trails backwards...
Step-by-step explanation:
When scientists face a new disease, the first steps they take is to find the cause of it. That include extensive body tissue and fluid testings. When they came to know about the infectious agents that is responsible for the disease, they took immediate measures to know the transmission vector of that particular disease. Extensive examination of several patients reveals the mode of transmission. This examination include their routinely habits, their recent exposure to something extraordinary etc...