Viral Exanthem: This is a common cause of rashes in children and adults, often presenting with a widespread rash on the chest, back, and head, accompanied by fever and other systemic symptoms. The rash can be caused by various viruses, including enteroviruses, adenoviruses, and parvovirus B19.

Eczema/Dermatitis: Atopic dermatitis or contact dermatitis can cause rashes on the chest, back, and head, especially if there's exposure to allergens or irritants. The rash is typically itchy and can vary in appearance.
Psoriasis: This chronic skin condition can cause red, scaly patches on the skin, including the chest, back, and scalp. It's often accompanied by itching and can be triggered by various factors.
Allergic Reactions: Allergic contact dermatitis or allergic reactions to medications can cause rashes on the chest, back, and head. The rash can be itchy, red, and blistering.

Meningococcemia: This is a life-threatening infection caused by Neisseria meningitidis, which can present with a rash on the chest, back, and head, along with fever, headache, and neck stiffness. The rash can be petechial or purpuric.
Rocky Mountain Spotted Fever: This tick-borne illness can cause a rash on the chest, back, and head, along with fever, headache, and other systemic symptoms. The rash can be petechial or maculopapular.
Stevens-Johnson Syndrome: This is a severe skin and mucous membrane disorder, often caused by medications or infections, which can present with a rash on the chest, back, and head, along with blisters, fever, and other systemic symptoms.

Kawasaki Disease: This rare condition can cause a rash on the chest, back, and head, along with fever, lymphadenopathy, and other systemic symptoms. It's more common in children and can lead to cardiac complications if left untreated.
Pityriasis Rubra Pilaris: This rare skin condition can cause a rash on the chest, back, and head, along with scaling, redness, and itching. It can be associated with other systemic symptoms and has a variable prognosis.