Viral Exanthem: This is the most common cause of fever with rash, especially in children. Viral exanthems, such as roseola or measles, often present with a characteristic rash and fever, along with other symptoms like cough or runny nose.

Bacterial Infections: Conditions like streptococcal pharyngitis or pneumonia can cause fever and rash. The rash may appear as a result of the infection itself or as a reaction to antibiotics.
Allergic Reactions: Allergic reactions to medications, foods, or environmental factors can cause fever and rash. These reactions can range from mild to severe and are often accompanied by other symptoms like itching or difficulty breathing.
Insect-Borne Illnesses: Diseases like Lyme disease or Rocky Mountain spotted fever can present with fever and rash after a tick bite.

Meningococcemia: This is a life-threatening condition caused by Neisseria meningitidis. It presents with fever, rash, and severe illness, often progressing rapidly. The rash can be petechial or purpuric and does not blanch under pressure.
Toxic Shock Syndrome: Caused by bacterial toxins, this condition is characterized by high fever, rash, and hypotension. It can be life-threatening if not recognized and treated promptly.
Septicemia: Bacteremia or septicemia can cause fever and rash, especially in immunocompromised patients. The rash may be a sign of disseminated intravascular coagulation (DIC) or another complication.

Kawasaki Disease: An autoimmune condition that affects children, characterized by fever, rash, and inflammation of the blood vessels. It's rare but important to diagnose early to prevent cardiac complications.
Erythema Multiforme: A skin condition characterized by target lesions, often triggered by infections or medications. It can be associated with fever and is considered rare.
Stevens-Johnson Syndrome: A severe skin and mucous membrane disorder, usually a reaction to medication or infection. It's rare and can be life-threatening, presenting with fever, rash, and significant morbidity.