Introduction. Scarlet fever is a bacterial illness that mainly affects children. It causes a distinctive pink-red rash. The illness is caused by Streptococcus pyogenes bacteria, also known as group A streptococcus, which are found on the skin and in the throat.s it's so contagious, scarlet fever is likely to affect someone in close contact with a person with a sore throat or skin infection caused by streptococcus bacteria. Outbreaks often occur in nurseries and schools where children are in close contact with one another.