Can the rash from roseola vary enoigh to be difficult to diagnose?

Yes: Yes, the differential diagnosis of roseola is wide and can include:rubella, rubeola, enterovirus, erythema infectiosum, or even scarlet fever. Seldom is anything cut and dry in medicine.

ROSEOLA: Caused by human herpes virus-6, 7. Age: younger than 3 yr, peak 6-15 mo. Starts with high fever 101-106 for 3-5 days, resolves abruptly, within 12-24h rose color rash appears as discrete, small (2-5 mm), slightly raised on trunk and then spreads to neck, proximal extremities; no itching or blistering; fades in 1-3 days. Some confusion with rash of rubella and measles.