As temperatures across the Earth rise, an invisible killer lurks — always ready to strike the unsuspecting. One of the deadliest side effects of climate change, heat stroke is becoming an alarmingly common phenomenon. But what exactly causes heat stroke, and why is it so dangerous? And most importantly, what can we do to prevent it? Thermal physiologist Dr Jason Lee talks about the workings of this hidden threat, plus some crucial tips to keep yourself safe in the blazing heat.