Some science behind the scenes
A Jarisch–Herxheimer reaction is a reaction to endotoxin-like products released by the death of harmful microorganisms within the body.
“Efficacious antimicrobial therapy results in lysis (destruction) of bacterial cell membranes, and in the consequent release into the bloodstream of bacterial toxins, resulting in a systemic inflammatory response.”
Or to put this in another way when the microbes die their bodies may end up in the blood stream and produce side effects.
Although Jarisch–Herxheimer reactions are ‘usually not life-threatening’, the side effects can be most unpleasant - fever, chills, hypotension, headache, tachycardia, vasodilation with flushing, myalgia (muscle pain), and rashes and other skin lesions. Overall it produces inflammation and a form of shock.
The Jarisch–Herxheimer reaction was first noticed in the treatment of syphilis, although when Jarisch and Herxheimer observed reactions in patients with syphilis treated with mercury, they were watching mercury poisoning. Heavy metal poisoning. Nevertheless the shock induced still has the same name.
There have been case reports of the Jarisch-Herxheimer reaction accompanying treatment of other infections, including Q fever, bartonellosis, brucellosis, trichinellosis, and African trypanosomiasis.
It should be treated as though the person is in shock. Sleep, warmth, comfort and reassurance as well as lots of warm drinks to help flush the system of the pathogens are often the most effective form of treatment.