We introduce a new class of physics-informed neural networks-EINN-crafted for epidemic forecasting. We investigate how to leverage both the theoretical flexibility provided by mechanistic models as well as the data-driven expressability afforded by AI models, to ingest heterogeneous information. Although neural forecasting models has been successful in multiple tasks, long-term predictions and anticipating trend changes remain open challenges. Epidemiological ODE models contain mechanisms that can guide us in these two tasks; however, they have limited capability of ingesting data sources and modeling composite signals. Thus we propose to supervise neural networks with epidemic mechanistic models while simultaneously learning their hidden dynamics. Our method EINN allows neural models have the flexibility to learn the disease spread dynamics and use auxiliary features in a general framework. In contrast with previous work, we not assume the observability of complete dynamics and do not need to numerically solve the ODE equations during training. Our thorough experiments showcase the clear benefits of our approach with other non-trivial alternatives.