 When Galen's animosity with the Roman medical practitioners became serious, he feared he might be exiled or poisoned, so he left the city.Rome had engaged in foreign wars in 161; Marcus Aurelius and his colleague Lucius Verus were in the north fighting the Marcomanni.During the autumn of 169 when Roman troops were returning to Aquileia, a great plague broke out, and the emperor summoned Galen back to Rome.