A human body digital twin (DT) is a virtual representation of an individual's physiological state, created using real-time data from sensors and medical test devices, with the purpose of simulating, predicting, and optimizing health outcomes through advanced analysis and modeling. The human body DT has the potential to revolutionize healthcare and wellness, but its responsible and effective implementation requires consideration of multiple intertwined engineering aspects. This perspective presents a comprehensive overview of the status and prospects of the human body DT and proposes a five-level roadmap to guide its development, from the sensing components, in the form of wearable devices, to the data collection, analysis, and decision-making systems. The article also highlights the necessary support, security, cost, and ethical considerations that must be addressed to ensure responsible and effective implementation of the human body DT. In the end, we provide a framework for the development and offer a unique perspective on the future of the human body DT, facilitating new interdisciplinary research and innovative solutions in this rapidly evolving field.