'diphtherial' originates from Modern Latin, specifically the word 'diphtheria', where the Greek root 'diphthera' meant 'prepared hide' (referring to the membranous patch seen in the disease).

'diphtherial' changed from New/Modern Latin 'diphtheria' by the addition of the English adjectival suffix '-al', producing the modern English adjective 'diphtherial'.

Initially the root referred to 'prepared hide' because of the membrane-like appearance; over time it became the name of the disease 'diphtheria' and then the adjective 'diphtherial' came to mean 'relating to diphtheria'.