The document is a World Health Organization publication about communicable disease surveillance and response systems. It explains that communicable disease surveillance is a core public health function used to collect, analyse and interpret health data so that outbreaks and other health threats can be detected early, monitored and responded to appropriately. The guide describes how surveillance systems help provide early warning of potential threats, support programme monitoring, enable outbreak detection and facilitate timely public health action to prevent disease spread. It also discusses the design and evaluation of surveillance systems and how the information they generate is used for decision-making in public health practice.