Differential Diagnosis for Rash and Fever in Adults

Single Most Likely Diagnosis

• Viral Exanthem: This is the most common cause of rash and fever in adults, often due to viral infections such as influenza, mononucleosis, or enterovirus. The rash can vary in appearance, and the diagnosis is often made based on clinical presentation and laboratory tests to identify the underlying virus.

Other Likely Diagnoses

• Bacterial Infections: Such as streptococcal or staphylococcal infections, which can cause a range of rashes, including impetigo, cellulitis, or abscesses, accompanied by fever.
• Allergic Reactions: Including drug reactions or contact dermatitis, which can present with a rash and fever, especially if there is a systemic response.
• Fungal Infections: Like ringworm or candidiasis, which can cause a rash and may be accompanied by fever, especially in immunocompromised individuals.

Do Not Miss Diagnoses

• Meningococcemia: A life-threatening infection caused by Neisseria meningitidis, characterized by a petechial or purpuric rash and high fever. Early recognition and treatment are crucial.
• Rocky Mountain Spotted Fever (RMSF): A tick-borne illness caused by Rickettsia rickettsii, presenting with fever, headache, and a characteristic rash. It is fatal if not treated promptly.
• Toxic Shock Syndrome: A severe condition caused by bacterial toxins, often associated with Staphylococcus aureus or Streptococcus pyogenes, characterized by high fever, rash, and multi-organ failure.
• Septicemia: Bacteremia leading to sepsis, which can present with a rash (e.g., petechiae) and fever among other symptoms, and requires immediate medical attention.

Rare Diagnoses

• Erythema Multiforme: An immune-mediated disorder characterized by target lesions and can be associated with fever, often triggered by infections or medications.
• Kawasaki Disease: Although more common in children, it can occur in adults, presenting with fever, rash, and other systemic symptoms, requiring prompt diagnosis and treatment to prevent cardiac complications.
• Syphilis: Secondary syphilis can cause a rash and fever, among other symptoms, and is an important consideration in sexually active adults, especially those with multiple partners or without consistent condom use.
• Lyme Disease: Caused by Borrelia burgdorferi, transmitted by ticks, and can present with a characteristic "bull's-eye" rash (erythema migrans) and fever, although the rash may not always be present.