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By Mayo Clinic staffOlder infants are at greatest risk of acquiring roseola because they haven't had time yet to develop their own antibodies against many viruses. While in the uterus, babies receive antibodies from their mothers that protect them as newborns from contracting infections, such as roseola. But this immunity fades with time. The most common age for a child to contract roseola is between 9 and 21 months.