First, the pathogens: Roseola in infants is caused by human herpesvirus type 6 infection, whereas measles is caused by the measles virus. Second, clinical manifestations: The clinical presentation of roseola includes high fever for three to five days followed by a rash. Typically, by the time the rash appears, the body temperature has already started to decline. The decline of fever concurrent with the appearance of the rash is a major diagnostic feature of roseola in infants. In contrast, measles typically involves high fever for three to four days before the onset of a rash. The rash tends to appear when the body temperature is at its peak, and it takes about 3-4 days for the rash to be fully present. The progression of the rash typically follows a sequence starting from the face, neck, and then the trunk, in a complete order. The primary difference between roseola and measles is the relationship between fever and body temperature: in roseola, the fever subsides and the rash appears after 4-5 days once the body temperature begins to decrease. In measles, the rash appears as the body temperature reaches a peak after 3-4 days of fever. This is the key distinction between roseola and measles.