From the description of symptoms by Greek physician Galen, which included coughing, fever, (blackish) diarrhea, swollen throat, thirstiness, modern experts identified the probable culprits of the Antonine Plague (A.D. 165–180) to be smallpox or measles. The disease likely started in China and spread to the West via the Silk Road. Roman troops first contracted the disease in the East before returning home. Striking a 'virgin population', the Antonine Plague had dreadful mortality rates; between one third to a half of the population, 60 to 70 million people, perished. Roman cities suffered from a combination of overcrowding, poor hygiene, and unhealthy diets. They quickly became epicenters. Soon, the disease reached as far as Gaul and mauled Roman defenses along the Rhine. The ranks of the previously formidable Roman army had to be filled with freed slaves, German mercenaries, criminals, and gladiators. It ultimately failed to prevent the Germanic tribes from crossing the Rhine. On the civilian side, the Antonine Plague created drastic shortages of businessmen, which disrupted trade, and farmers, which led to a food crisis. An economic depression followed and government revenue fell. Some accused Emperor Marcus Aurelius and Co-emperor Lucius Verus, both of whom victims of the disease, of affronting the gods while others blamed Christians. Yet the Antonine Plague strengthened the position of the monotheistic religion of Christianity in a heretofore polytheistic society as Christians won public admiration for their good works. Ultimately it was the Roman army, Roman cities, the size of the empire and its trade routes, without which Roman power and influence would not exist, that facilitated the spread of the disease. The Antonine Plague is considered by some historians as a useful starting point for understanding the decline and fall of the Western Roman Empire. It was followed by the Plague of Cyprian (A.D. 249–262) and the Plague of Justinian (541-542). Together, they cracked the foundations of the Roman Empire.