Your dog can’t tell you when she’s feeling sick, but even so, you know. She moves slowly, she doesn’t eat, she sleeps a lot, she curls up in a corner by herself. “Sickness behavior” is shared by all mammals, and in humans it’s been shown to include fatigue, cognitive problems, body aches and pains and disturbed sleep. These symptoms sound very familiar to patients with ME/CFS and fibromyalgia, and recently several researchers have suggested that many core symptoms of these illnesses could be the result of sickness behaviour biology gone wrong.
Sickness behaviour is normally a good thing, helping us survive by resting to fight off infection. But it evolved as a short-duration response, and may be harmful if it sets in for the long-term, perhaps playing a role in ME/CFS.
In other words we got sick but the ‘sickness switch’ was never turned back off.