Seemingly trivial modifications to the classical model of contagion spreading can dramatically alter its phenomenology. For example, discontinuous phase transitions can occur due to complex or interacting contagions, accelerating transmission and hysteresis loops can occur when individuals modify their behaviour after becoming infectious, and double phase transitions can emerge in the presence of asymmetric percolation. In this talk, I will present recent theoretical work on the affect of behaviour on contagion spreading and discuss empirical support for these new models. Our findings demonstrate the inherent complexity of biological contagion and we anticipate that our methods will advance the emerging field of disease forecasting.