Henipaviruses at the threshold: preparedness in the era of recurrent spillover.

Henipaviruses remain high-consequence zoonotic pathogens characterized by recurrent spillover, severe disease, and limited but consequential human-to-human transmission. In this commentary, we frame henipaviruses as "threshold pathogens": agents repeatedly approaching, but not yet consistently crossing, the boundary into sustained community transmission. Expanding viral diversity, widening ecological interfaces, and episodic outbreaks challenge conventional preparedness models, particularly for surveillance, countermeasure evaluation, and regulatory decision-making. We argue that future preparedness should move beyond outbreak counting towards integrated One Health surveillance, adaptive evidence-generation pathways, and trial-ready countermeasure strategies suited to pathogens with irregular emergence and high public health consequences.