Viral Exanthem (Non-specific viral rash): This is a common condition in children and adults, characterized by a rash that can resemble measles, but without the typical symptoms of measles such as fever. The presence of URI symptoms supports a viral etiology.

Roseola: Although typically associated with a high fever that precedes the rash, some cases can present with mild or no fever, especially in adults. The rash and URI symptoms could align with a roseola infection, particularly if the fever was missed or very mild.
Erythema Infectiosum (Fifth Disease): Caused by Parvovirus B19, this condition often presents with a "slapped cheek" appearance followed by a lacy rash on the body. While it usually includes mild fever, some cases might not, and URI symptoms can sometimes accompany the rash.
Allergic Contact Dermatitis or Irritant Contact Dermatitis: These conditions can cause rashes that might resemble measles in their distribution and appearance, especially if the allergen or irritant causes a widespread reaction. URI symptoms might be coincidental or related to an allergic reaction.

Secondary Syphilis: This condition can cause a rash that might resemble measles, along with various other symptoms including URI-like symptoms (e.g., sore throat). It's crucial to consider syphilis due to its serious long-term consequences if left untreated.
Meningococcemia: Although typically associated with fever, meningococcemia can cause a rash (petechial or purpuric) and might occasionally present without fever early in the course. Given its high mortality rate, it's essential to consider this diagnosis, even if the presentation is atypical.
Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS): This is a severe drug reaction that can cause a measles-like rash, fever (which might be absent in some cases), and involvement of internal organs. URI symptoms could be part of the systemic manifestation.

Infectious Mononucleosis (Mono): Caused by Epstein-Barr virus, mono can occasionally present with a rash, which might resemble measles, especially if the patient is taking ampicillin. URI symptoms are common in mono.
Kawasaki Disease: Typically presents with fever, but in its early stages or atypical forms, it might not. The rash and URI symptoms could fit, although other criteria like conjunctivitis, lymphadenopathy, and changes to the lips and hands would also need to be considered.
Scarlet Fever: While classically associated with a distinct rash and fever following a streptococcal pharyngitis, some cases might present with a measles-like rash and URI symptoms without prominent fever.