Scarlet fever is an infectious disease caused by toxin-producing strains of the bacteria Streptococcus pyogenes, also known as group A streptococcus (GAS). GAS can colonise the throat or the skin. Scarlet fever occurs most commonly in association with pharyngitis, but may also result from infection at other sites, including wound infection and puerperal sepsis.

In England, scarlet fever is a notifiable disease and typically around 3,000–4,000 cases are diagnosed each year. Recently, however, there has been a substantial increase in cases, with more than 14,000 cases notified in 2013–2014.