JERUSALEM, July 3 -- For decades, the working model of antiviral immunity in animals came down to a simple equation: viral invasion triggers proteins that switch on the immune system, which then clears the infection. A sea anemone the length of a fingernail has just overturned that picture.

A study published in Nature Ecology & Evolution shows that Nematostella vectensis, a small coastal sea anemone used widely in evolutionary biology research, fights viral infections not by activating its immune system but by depending on a protein that keeps it intentionally suppressed. That suppression, it turns out, is the mechanism of survival.

The finding disrupts a core assumption about animal immunity: that all animals inherited some version of ...