It's an illness that's often described as 'hidden'. Thousands of kids around the world, and here in New Zealand, too unwell to go to school or even get up off the bed.
It's complex and misunderstood, there's even debate over what to call it. Some know it as Chronic Fatigue Syndrome, others M.E.
Freelancer Felicity Monk wrote a report about the illness for The Wireless and joins us to tell us about the stories she heard.