Viral Exanthem: This is the most common cause of fever with rash, especially in children. Viral exanthems are caused by various viruses and often present with a characteristic rash and fever, along with other symptoms like cough, runny nose, or sore throat.

Bacterial Infections (e.g., Scarlet Fever, Impetigo): These can cause fever and rash, often with additional symptoms like a sore throat or skin lesions. The rash in bacterial infections can be distinctive, such as the sandpapery rash of scarlet fever.
Allergic Reactions: Allergic reactions to medications, foods, or environmental factors can cause a rash and fever. The rash can range from mild to severe and may be accompanied by other symptoms like itching or difficulty breathing.
Insect-Borne Illnesses (e.g., Lyme Disease, Dengue Fever): Depending on the geographic location, insect-borne illnesses can cause fever and rash. For example, Lyme disease is characterized by a distinctive "bull's-eye" rash, while dengue fever can cause a petechial rash.

Meningococcemia: This is a life-threatening infection caused by Neisseria meningitidis. It can present with fever, rash (often petechial or purpuric), and signs of meningitis or sepsis. Early recognition and treatment are crucial.
Rocky Mountain Spotted Fever (RMSF): Caused by Rickettsia rickettsii, RMSF is characterized by fever, headache, and a rash that typically appears a few days after the onset of fever. It's essential to consider RMSF in patients with a history of tick exposure.
Toxic Shock Syndrome: This is a severe condition caused by bacterial toxins, often associated with Staphylococcus aureus or Streptococcus pyogenes. Symptoms include high fever, rash, hypotension, and involvement of multiple organ systems.

Kawasaki Disease: An acute febrile illness of childhood characterized by fever, rash, conjunctivitis, and changes in the lips and oral cavity. It can lead to coronary artery aneurysms if not treated promptly.
Erythema Multiforme: A skin condition characterized by target lesions, often triggered by infections or medications. While not usually life-threatening, it can be a sign of a significant underlying condition.
Syphilis: Secondary syphilis can cause a rash and fever, among other symptoms. It's essential to consider syphilis in the differential diagnosis, especially in sexually active individuals or those with a history of high-risk behaviors.