Jarisch Herxheimer reaction

It refers to the transient inflammatory response seen when administration of antibiotics results in large scale killing of microorganisms and release of endotoxins. The rate of release of endotoxins is much greater than the rate at which the body can detoxify them. This results in manifestations such as fever, myalgia, headache. 

eg: may be found during treatment of syphilis, leptospirosis