There are many mysteries around fever, but we do know that all mammals, some birds and even a few invertebrate and plant species feel fever's heat. It has persisted for over 600 million years of evolution. But it has a significant cost: for every degree increase in temperature, there's a 12.5% increase in energy required. So, why and how does your body produce a fever? Christian Moro investigates. [Directed by Artrake Studio, narrated by Addison Anderson].