'filariasis' originates from New Latin/medical formation, combining 'filaria' (a term for thread-like worms) with the Greek suffix '-iasis' meaning 'disease or pathological condition'.

'filaria' itself derives from Latin 'filum' meaning 'thread'; the modern medical term 'filariasis' was formed in the 19th century by combining that root with the Greek-derived disease suffix '-iasis' to denote infection by thread-like worms.

Initially, the root referred simply to 'thread' (the appearance of the worms); over time the compounded medical term came to mean the specific diseases caused by those thread-like filarial worms, especially infections of the lymphatic system.