"Sickness behavior" is when the immune system goes into a taxing response mode to an infection, etc. causing fatigue, fever, depression, and more. An essentially chronic form is chronic fatigue syndrome (CFS). One possible viral of bacterial infection trigger for the condition, hypothesized by an article published in 2013, is one of the vagus nerve(2).
Sources:
1. Ram, Nanditha. "The nervous system, migraine and yoga therapy." Natural.News, Natural News Features, LLC, 19 February 2016, http://www.natural.news/2016-02-19-the-nervous-system-migraine-and-yoga-therapy.html
2. VanElzakker, Michael B. "Chronic fatigue syndrome from vagus nerve infection: a psychoneuroimmunological hypothesis." Med Hypotheses, vol. 81, no. 3, pp. 414-23. GreenMedInfo, http://www.greenmedinfo.com/article/chronic-fatigue-syndrome-vagus-nerve-infection-psychoneuroimmunological-hypoth, Abstract.