'infective' originates from Latin, specifically the word 'infectus' (the past participle of 'inficere'), where 'in-' meant 'in' or 'into' and the root related to 'ficere' (cf. 'facere') meant 'to make or do'.

'infectus' in Latin passed into Medieval Latin and was influential on Old French/Anglo-Norman forms; English formed the adjective by relation to the verb 'infect' plus the adjectival suffix '-ive', yielding the modern English 'infective'.

Initially it carried senses like 'stained, tainted, or poisoned' in Latin contexts, but over time it evolved to the modern medical/biological meaning 'capable of causing infection'.