The incubation period for scarlet fever has a fairly wide range from about 12 hours to seven days. Individuals are contagious during this first subclinical or incubation period and during the acute illness. The primary strep infection is the contagious . Jun 03, · The incubation period for scarlet fever ranges from 12 hours to 7 days. Patients are contagious during the acute illness and during the subclinical phase.