The primary cause of scarlet fever is the exotoxin released by the streptococcus pyogenes. This is the same type of bacteria that causes strep throat and rheumatic fever. Therefore if you have been having a persistent strain of strep throat, you should get a differential diagnosis.
It is also very infectious and can spread through the air, from an infected person via the droplets expelled by the infected person. It is, therefore, very important to recognize the signs and symptoms of scarlet fever, get diagnosed and medicated quickly in order to prevent the spread of infection.