After 3 days of fever a patient developed macular erythematous rash that lasted for 48 hrs on the 4th day of fever. The most likely diagnosis is:

Exanthem subitum/roseola infantum/6th disease is due to infection by HHV - 6. It is characterised by fever and subsequent rash. Approximately 10-20 % febrile seizures without rash are caused by HHV-6. Ref: Harrison's Principles of Internal Medicine, 14th Edition, Page 1159 and 15th Edition, Page 96