antifungal
An antifungal is a drug that kills or holds back fungi, the organisms behind infections from athlete's foot and thrush to life-threatening bloodstream and lung disease. Fungi pose a special challenge: like us, they are eukaryotes, so their cells are biochemically much more similar to ours than bacteria are, leaving fewer safe targets to attack.
Most antifungals exploit the one big difference fungi do have: their cell membranes use ergosterol where ours use cholesterol. Azoles such as fluconazole block the enzyme that makes ergosterol; polyenes such as amphotericin B punch holes in the ergosterol-containing membrane; and echinocandins take a different route, blocking synthesis of the fungal cell-wall component glucan, which human cells lack entirely and which gives them an unusually clean target.
Because fungi are so cell-similar to us, systemic antifungals tend to be more toxic than antibiotics; amphotericin B in particular is notorious for kidney damage and infusion reactions. Azoles also strongly inhibit human cytochrome P450 enzymes, causing many drug interactions. Superficial infections of skin and nails can often be treated topically with minimal risk, but deep or invasive fungal disease usually demands carefully monitored systemic therapy.
Oral thrush in an otherwise healthy person is treated with a topical antifungal like nystatin, whereas invasive candidaemia needs an intravenous echinocandin.
Topical for superficial, systemic for invasive — matching route to depth of infection.
Amphotericin B has earned the grim nickname amphoterrible for its toxicity; modern lipid formulations were developed largely to make it more tolerable.