Before reaching Naples in the summer of 1884, cholera had long been expected. The endemic home of the disease was in the Ganges-Brahmaputra Delta in West Bengal. Until the nineteenth century, the Vibrio cholerae had proven too perishable to survive the journey from the sub-continent to western Europe. In the new century, however, the steamship and the railroad transformed the length of voyages. At the same time the growing economic and military presence of the British in India multiplied the contacts between East and West. The medical consequence was that cholera for the first time acquired the means of spreading far beyond its original Asiatic habitat.

Six times between 1830 and the First World War cholera spread from Bengal to the West along two great microbial highways. To the north it followed the trade routes to the Punjab, Afghanistan and Persia. Having crossed the Caspian Sea, the germ passed into Europe at Astrakhan and Orenburg, and then journeyed up the Volga to Moscow and the heart of the continent. Alternatively, cholera travelled to the south – overland to Bombay and Madras, then by sea to Ceylon and Jedda. It then accompanied the pilgrims to Mecca, and dispersed with them to Egypt and the Mediterranean seaports.

The renewed menace to Naples began in 1881, when the bacterium began its fifth and penultimate voyage to Europe. In 1883 it reached Egypt with Muslim pilgrims and with Indian troops serving in the British army of occupation. The frightful carnage in Egypt, where 60, 000 people perished in the course of a few months, raised the alarm throughout the Mediterranean.