A fever is one sign of the body’s defense response. A fever occurs when a pyrogen, a toxin, or cytokine produced by an infectious agent, human tissue, or a medication acts on the anterior hypothalamus to increase the body’s temperature set point. Because fever can occur as a nonspecific response associated with a variety of illnesses, both infectious and noninfectious, other clues are used to determine the underlying cause. Oftentimes, a concurrent rash can help clinch the diagnosis. Fever and rash can occur as a result of an infection, a reaction to a medication, or a systemic illness. A thorough and complete history and physical examination is essential to determine the underlying etiology.

There are a number of different types of rashes that can occur with fevers. Observing the specific morphology, associated clinical features, distribution, and natural history of the eruption can aid in the diagnosis. Occasionally, fever and rash can be the presenting features of a life-threatening illness and these require immediate intervention (see GENERALIZED ERUPTIONS). In this chapter, some of the most common rashes that occur with fever are outlined by morphology of the eruption, their typical distribution, natural history, and associated symptoms are presented to serve as an aid in the diagnosis.

Morbilliform eruptions are composed of pink to red discrete macules and slightly raised papules that oftentimes have a peripheral blanched halo. The most common causes of fever and morbilliform eruptions can be seen in the setting of an infection, a drug reaction, or a systemic illness; the most common associations are listed below.