Viral Exanthem: This is a common condition characterized by a rash that can start on the chest and spread to other parts of the body. It is often associated with viral infections and can be accompanied by fever and other systemic symptoms. The rapid spread of the rash over one day is consistent with a viral etiology.

Allergic Contact Dermatitis: This condition occurs when the skin comes into contact with an allergen, leading to a rash. It can start on the chest if the allergen is in contact with this area and can spread if the allergen continues to come into contact with other parts of the skin.
Irritant Contact Dermatitis: Similar to allergic contact dermatitis but caused by an irritant rather than an allergen. It can also start on the chest and spread, depending on the exposure to the irritant.
Erythema Multiforme: An immune-mediated condition that can be triggered by infections or medications, characterized by target-like lesions that can appear on the chest and spread to other areas.

Toxic Epidermal Necrolysis (TEN): A severe skin condition usually triggered by medications, leading to widespread skin necrosis and detachment. It can start with a rash on the chest and rapidly progress, making early recognition critical for survival.
Stevens-Johnson Syndrome (SJS): Similar to TEN but less severe, it is also a medication-induced condition that can start with a rash on the chest and spread, involving mucous membranes and potentially leading to significant morbidity.
Meningococcemia: A bacterial infection that can cause a rash, often starting on the chest, which can rapidly progress and lead to sepsis and death if not promptly treated.

Pityriasis Rubra Pilaris: A rare skin disorder characterized by constant scaling, redness, and thickening of the skin, which can start on the chest and spread.
Kawasaki Disease: A rare condition that mainly affects children, characterized by fever, rash, swelling of the hands and feet, and other symptoms. The rash can start on the chest and spread to other areas.