What is Scarlet fever? How long after onset of a strep throat infection does Scarlet fever emerge?

What is Scarlet fever? How long after onset of a strep throat infection does Scarlet fever emerge? What is the pattern of the rash in Scarlet fever? What is special about the strains that can cause Scarlet fever as compared to those that just cause strep throat? What bacterial product is responsible the diffuse rash? How is the gene encoding that bacterial product carried on the bacterial chromosome? Why is the rash not localized to the site of infection? What is the treatment for Scarlet fever? How many cases/yr are there of Scarlet fever in the U.S.? How does that compare to ~50 years ago?