Skin Infections & Infestations

Antifungals in Dermatology: Tinea, Candida and the Nail Problem

A ring on the skin, a white curd in a groin fold, a thickened crumbling toenail — superficial fungal infections are among the most common things a dermatologist sees, and among the most over-treated with the wrong drug for the wrong length of time. The organisms are simple: dermatophytes that eat keratin, Candida that loves warm damp folds, and Malassezia that lives in every sebaceous follicle. The pharmacology is elegant, because every antifungal attacks the same weak point — the fungal cell's own membrane. Understanding that one membrane, and why keratin makes the nail and scalp so stubborn, is the whole chapter.

Bacterial Skin Infections: From Impetigo to Cellulitis and MRSA

Almost every bacterial skin infection is a story about two organisms — Staphylococcus aureus and Streptococcus pyogenes — and one broken barrier. The skin is a wall; when a scratch, a bite, an insect sting or a patch of eczema breaches it, these two commensals turn invader. The pharmacology that follows is not about reaching for the strongest antibiotic — it is about matching the drug to the depth of the infection, the likely organism, and, increasingly, to whether that organism has learned to shrug off our first-line penicillins. Get the severity and the resistance question right, and most of these infections are simple to cure. Get them wrong, and a red leg becomes a hospital admission.

Viral Skin Disease: Herpes, Warts and Molluscum

Bacteria on the skin can be met head-on: an antibiotic that poisons a target the bacterium has and we don't. Viruses are harder. They hide inside our own cells and borrow our machinery, so a drug that kills the virus risks killing the cell around it. The elegant answer for herpes was to design a drug that only switches on inside an infected cell. The stranger answer for warts was to stop attacking the virus altogether — and instead wake up the immune system that should have cleared it in the first place. This chapter is about both tricks.

Scabies and Lice: Killing the Parasite on the Skin

Some skin diseases are about inflammation gone wrong. This one is about a living animal — a mite the size of a full stop that tunnels through the outer skin and lays its eggs there. The treatment is refreshingly literal: you apply a chemical that paralyses the parasite and it dies. And yet the single most common reason a "failed" scabies case walks back into clinic is not a surviving mite at all — it is an immune system still raging at a parasite that is already dead. Understanding that gap between killing the mite and calming the skin is the whole art of treating infestations.