Sounds like depression to me

A signaling system in the brain previously shown to regulate sleep is also responsible for inducing lethargy during illness.

To determine the cause of illness-induced lethargy, Marks and colleagues studied the brains of rats, the neuroactivity of which closely mimics human brains. They determined that acute and chronic inflammation-induced lethargy is brought about by a specific population of inflammation-sensitive neurons located near the neurotransmitter system that controls physical activity and arousal, known as the orexin system. When the researchers injected the rats with orexin, they were able to restore orexin signaling and, thus, restore motivated behaviors and movement.