Viral effectors trigger innate immunity across the tree of life.

Viruses are ubiquitous biological entities that exert immense selective pressures on their hosts, driving the evolution of diverse innate immune mechanisms across all domains of life. While innate immunity has historically been studied in eukaryotes, recent discoveries of bacterial defence systems against phages reveal striking parallels between prokaryotic and eukaryotic immunity. A key principle of antiviral defence conserved from bacteria to humans is pattern recognition, where virus-associated molecular patterns trigger immune responses. In addition to pattern recognition, effector-triggered immunity (ETI) involves the detection of pathogen-induced perturbations of host cell pathways. ETI, initially described in plants and later in animals, was recently shown to have parallels in bacterial immunity as well. In this perspective, we explore how viral infections in prokaryotic and eukaryotic cells manipulate comparable host pathways, creating molecular signatures that are recognized by distinct immune systems. By examining the shared features and mechanisms underlying ETI, we illuminate its role as a core principle of host-pathogen interactions across the tree of life.This article is part of the discussion meeting issue 'The ecology and evolution of bacterial immune systems'.