The agent worked, but too well: 99.9% of the population became so lethargic that they stopped working, talking and, eventually, eating and moving.
en.wikipedia.org Approximately 50% of affected puppies are lethargic and depressed, as in this case.
en.wikipedia.org Infected amphibians can display a wide variety of symptoms, usually including lethargic and abnormal behavior, convulsions, peeling skin, ulcers, and hemorrhaging; eventually resulting in death.
en.wikipedia.org Some individuals are very aggressive and will strike furiously, while others seem lethargic and do not even attempt to rattle.
en.wikipedia.org For a period after this he became very lethargic and reclusive, numbed by his wife's death.
en.wikipedia.org