Scarlet fever is a syndrome characterized by a blanching, erythematous, maculopapular rash often described as sandpaper-like, strawberry tongue, and exudative pharyngitis (see Image. Scarlet Fever). The causative organism, Streptococcus pyogenes (Lancefield Group A Streptococcus, or Strep A), is a gram-positive bacterium adapted to humans. This organism grows in pairs and chains and is responsible for a wide spectrum of infections, ranging from superficial to deep and invasive conditions, including cellulitis, pharyngitis, erysipelas, toxic shock syndrome, and necrotizing fasciitis.

Strep A produces streptococcal pyrogenic exotoxins (SPEs), which act as superantigens released during infection. These exotoxins are the primary cause of the erythematous rash associated with scarlet fever. Strep A bacterial pharyngitis and scarlet fever most commonly affect school-aged and adolescent children due to higher transmissibility in school settings. However, these infections can also occur in other age groups, particularly in crowded environments such as households and nursing homes.

Scarlet fever caused by Strep A infections can occur at any age. Although it is most commonly associated with Strep A pharyngitis, it may also develop with other Strep A infections, whether invasive or noninvasive, such as erysipelas or necrotizing fasciitis. Historically, Strep A serotypes have displayed cyclic epidemiological patterns. Notably, Strep A is among the few bacteria that produce superantigen exotoxins, which are exceptionally potent T-cell activators. Strep A superantigens, also known as erythrogenic or scarlet fever toxins, are responsible for the characteristic erythematous, sandpaper-like rash and strawberry tongue observed in scarlet fever. Superantigen genes, such as speA, speC, and ssa, enhance the fitness and virulence of Strep A, contributing to the development of invasive disease.

Scarlet fever epidemics and invasive Strep A infections were common in the 19th century. Although the prevalence of scarlet fever declined in the 20th century, a resurgence of Strep A infections occurred in the 1980s. Over the past decade, more virulent epidemic strains of Strep A have emerged, leading to increased Strep A infections and scarlet fever. Suppurative and nonsuppurative complications can arise from Strep A infections, including rheumatic heart disease and poststreptococcal glomerulonephritis. Prompt treatment of acute infections is essential to prevent these complications.