War over Armenia broke out again in 161, when Vologases IV defeated the Romans there, captured Edessa and ravaged Syria. In 163 a Roman counter-attack under Statius Priscus defeated the Parthians in Armenia and installed a favored candidate on the Armenian throne. The following year Avidius Cassius invaded Mesopotamia, winning battles at Dura-Europos and Seleucia and sacking Ctesiphon in 165. An epidemic which was sweeping Parthia at the time, possibly of smallpox, spread to the Roman army and forced its withdrawal; this was the origin of the Antonine Plague that raged for a generation throughout the Roman Empire. In 195–197, a Roman offensive under the Emperor Septimius Severus led to Rome's acquisition of northern Mesopotamia as far as the areas around Nisibis, Singara and the third sacking of Ctesiphon. A final war against the Parthians was launched by the Emperor Caracalla, who sacked Arbela in 216. After his assassination, his successor, Macrinus, was defeated by the Parthians near Nisibis. In exchange for peace, he was obliged to pay for the damage caused by Caracalla.