The main role of surveillance in epidemiology is monitoring disease trends in populations. This process involves the continuous, systematic collection, analysis, and interpretation of health data, which enables public health officials to detect outbreaks, assess the impact of interventions, and guide resource allocation. Surveillance is essential for identifying emerging health threats and controlling the spread of diseases at a population level. It does not involve direct patient care, drug development, or surgical procedures, which are outside its core function. Effective surveillance ultimately improves public health outcomes by informing timely decision-making.

Reference: Epidemiology, Leon Gordis, 5th Edition.