Mesopotamia is modern day Iraq. In 1914, that state didn’t exist, being ruled by the Ottoman Empire. The British campaign against the Turks there began with the occupation of Basra in November 1914. In the beginning it was run by the India Office, and the British commitment was to remain overwhelmingly from Indian Army soldiers. European formations were in the minority.
It was a hostile environment, overwhelmingly hot for much of the year. Disease rose and spread easily. The Turks fought hard. Communications were poor, and logistics difficult. Evacuation of wounded and sick to distant hospitals and hospital ships took much too long. In 1916 there was a disastrous surrender of a large number of British troops at the Siege of Kut.
It was a shock, and organisation improved after it. An inquiry, the Mesoptamian Commission, reported in June 1917. Mesopotamia remained a difficult posting, and tended to be viewed as a sideshow by the public. When the figures were totalled up at the end of the war the number of soldiers who died of sickness slightly exceeded those who were killed, 12,678 versus 11,012.
Few Shetlanders were involved in Mesopotamia. Unsurprisingly though the three listed as dying there in the Roll of Honour, died of heatstroke, malaria, and dysentery. Four are listed as serving. One was invalided out in 1919, another two were wounded. William Laurenson, from Minn, Burra, with the Royal Lancashire Regiment, is listed as serving there for four years, and seems to have emerged unscathed.