= Solution
\b[The historical common principle was to identify and destroy a superficial fungal parasite, remove material harbouring it and prevent reinfection. Modern diagnosis preserves this principle for genuine <fungal infection>, but rejects its indiscriminate application to “tinea decalvans”.] The three labels must therefore be discussed separately.
In <pityriasis versicolor>, look for finely scaling patches of altered <skin pigmentation>, commonly on the trunk. Examine superficial <skin> scrapings by <microscopy> when uncertain. Historically, washing, removal of loose scale and a topical “parasiticide” aimed to reach the organism in the outer <epidermis>. Today <pityriasis versicolor> is associated with overgrowth of <Malassezia> <yeasts> already present on the <skin>; it is not ordinary contagious <ringworm> and is not evidence of poor cleanliness.<Pityriasis versicolor by Nottinghamshire Area Prescribing Committee>[Clinical description and Patient advice; normal skin yeast, noncontagious condition]{ref} Topical <ketoconazole> or <selenium sulfide> can control it. Extensive or recurrent disease sometimes needs other treatment. Loss of scale and control of the <yeast> need not immediately restore <skin pigmentation>; residual colour can persist for weeks or months. A <Wood's lamp> can assist examination, but is not a substitute for the whole clinical assessment.<Tinea versicolor: Diagnosis and treatment by the American Academy of Dermatology>[Diagnosis; topical treatment; Outcome]{ref}
“Tinea decalvans” is the critical historical trap. Some nineteenth-century writers interpreted patchy baldness as parasitic and advocated extraction of affected hairs followed by prolonged topical treatment. A January–June 1867 digest explicitly calls the condition contagious and reports a <mercury(II) chloride> preparation. That is evidence of a contemporary belief, not evidence that the proposed organism caused every case of patchy baldness.<Half-yearly abstract of the medical sciences (January–June 1867)>[Article on Tinea decalvans immediately preceding Art. 65; report from Medical Press and Circular, 10 April 1867]{ref} The label often covered what is now called <alopecia areata>, an immune-mediated form of <hair loss>. Smooth bald patches require differentiation from scaly patches with broken hairs and demonstrated <fungal infection>. Confirm the diagnosis before treating. In established <alopecia areata>, a period “parasiticide” is inappropriate: observation or diagnosis-specific treatment, including local <corticosteroids> in suitable cases, is considered instead. Neither <mercury(II) chloride> nor destructive epilation is a modern treatment for <alopecia areata>.<Alopecia areata: Diagnosis and treatment by the American Academy of Dermatology>[Diagnostic assessment; limited recent patches; topical and injected corticosteroids]{ref}
For contagious <ringworm>, establish the site and extent. An enlarging scaly ring on bare <skin> suggests <tinea corporis>; scaly bald areas with broken hairs suggest <tinea capitis>. Examine scrapings or hairs by <microscopy> and, where appropriate, <fungal culture>. Clean clothing, towels, combs and brushes; avoid sharing them and investigate relevant human or animal sources. These measures address transmission, while an <antifungal drug> treats the affected tissue.<Ringworm by WHO>[Diagnosis; Treatment; Prevention, especially shared personal items and infected animals]{ref} Limited <tinea corporis> can often be treated topically. <Tinea capitis> needs an oral <antifungal drug>, such as <griseofulvin> or <terbinafine> chosen for the circumstances, because a cream alone does not clear the infected hair-bearing site. An adjunctive shampoo can reduce spread but does not replace systemic treatment. Unsupervised <corticosteroid> cream can mask or worsen <ringworm>.<Treatment of Ringworm by CDC>[Ringworm on the skin; Ringworm on the scalp; Things to avoid]{ref}<Ringworm: Diagnosis and treatment by the American Academy of Dermatology>[Scalp; antifungal shampoo as an adjunct and prevention of household reinfection]{ref}
Thus the useful unifying idea is \b[cause-directed treatment that reaches the affected tissue], with transmission control when infection is transmissible. It does not follow that all three nineteenth-century names denote the same class of disease, or that every bald patch requires killing a parasite.
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