Viral Exanthem: This is a common cause of a widespread rash, often seen in children but can occur in adults as well. It's usually associated with a viral infection and can be accompanied by fever and other systemic symptoms.

Atopic Dermatitis (Eczema): A chronic condition characterized by itchy, inflamed skin that can spread over the body. It often starts in childhood but can persist into adulthood.
Contact Dermatitis: An allergic reaction or irritation caused by substances coming into contact with the skin, leading to a rash that can spread if the allergen or irritant is widespread.
Drug Rash: Many medications can cause a rash as a side effect, which can appear anywhere on the body and spread.
Psoriasis: An autoimmune condition that speeds up the life cycle of skin cells, causing cells to build up rapidly on the surface of the skin. The extra skin cells form scales and red patches that are itchy and sometimes painful.

Toxic Epidermal Necrolysis (TEN) and Stevens-Johnson Syndrome (SJS): These are severe skin conditions usually triggered by medications, leading to widespread skin necrosis and detachment. They are medical emergencies.
Meningococcemia: A severe infection caused by Neisseria meningitidis, which can cause a rash that looks like pinpoint red or purple spots and can quickly progress to a more severe disease.
Septicemia: Bacteria in the blood can cause a rash among other symptoms, and it's a life-threatening condition requiring immediate medical attention.

Pityriasis Rubra Pilaris: A rare skin disorder that causes constant inflammation and scaling (exfoliation) of the skin, often starting on the face and spreading.
Kawasaki Disease: An illness that affects children and causes inflammation in the walls of medium-sized arteries throughout the body, often accompanied by a rash, among other symptoms.
Erythema Multiforme: A skin condition characterized by target lesions, which can be triggered by infections or medications, and can sometimes spread over the body.