Favus, also termed tinea favosa, is a chronic inflammatory dermatophytic infection usually caused by Trichophyton schoenleinii. [1, 2, 3] Rarely, favus is caused by Trichophyton violaceum, Trichophyton mentagrophytes var quinckeanum, or Microsporum gypseum. Favus typically affects scalp hair but also may infect glabrous skin and nails. The causative agent of mouse favus is T mentagrophytes var quinckeanum, also termed Trichophyton quinckeanum, which can cause favus in humans, although rarely. [4, 5]
Favus is a superficial dermatophyte infection usually caused by T schoenleinii and characterized by scutula. In most patients, favus is a severe form of tinea capitis; however, it may occur, although rarely, as onychomycosis, tinea barbae, or tinea corporis. Rarely, Tviolaceum, Trichophytonverrucosum, Microsporum audouinii, Microsporum gallinae, M gypseum, and Microsporum canis have been linked with favus. 
Favus is 1 of 3 primary patterns of hair infection (ectothrix, endothrix, favus). Typically, hair is not as heavily infected as in trichophytosis caused by Trichophyton tonsurans. Hair is able to grow, and frequently, long hairs are observed in the disease state. The most characteristic feature is the formation of air spaces between hyphae within the infected hair. These air spaces (air tunnels) form as a result of autolysis of the hyphae. Arthroconidia rarely are seen within the hair. Such infected hair commonly is termed favus-type hair. In the sera of patients, antibodies to causative fungi are found by charcoal agglutination and immunodiffusion assay; however, the exact role of antibodies is not clear. 
A survey of patients with tinea capitis in Karachi, Pakistan aged 1-14 years showed the following clinical types: gray patch in 71 (35.1%), black dot in 63 (31.2%), kerion in 50 (24.8%), favus in 10 (5%), diffuse pustular in 6 (3%), and diffuse scale in 2 (1%) patients. 
Favus is uncommon in the United States, although foci have been described in past decades in rural areas of West Virginia, New York, Kentucky,  and Arkansas. Favus often is seen in geographic regions where lifestyles are associated with malnutrition, neglect, and poverty.
Foci of favus have been seen worldwide, including Poland,  Southern and Northern Africa,  Pakistan,  the United Kingdom, Australia, South America (Brazil), [13, 14] Canada (Quebec), [15, 16] and the Middle East.  Favus had been the predominate form of tinea capitis in the Nanchang region of southern China, with a 1965 prevalence of 3.4% of the population. However, it has been successfully controlled, decreasing its prevalence to 0.01% by 1977, with favus now replaced by the black dot form of tinea capitis. 
Favus shows no racial or ethnic preference.
Both females and males may be affected equally; however, some report a slight predominance of female patients with favus.
Favus appears in both children and adults. Favus usually is acquired during childhood or adolescence and typically persists into adulthood.
Favus shows no tendency to resolve spontaneously. Cicatricial alopecia with skin atrophy is a common feature of long-lasting disease. In these patients, both scalp and glabrous skin often are affected. Permanent alopecia with scarring often follows favus, which is a chronic disfiguring infection.