Green’s Dictionary of Slang

(Anglo-Ind. milit.) cholera morbus.

1817		in J. Shipp Memoirs (1829) II 37: We are all pretty well, only the regiment is sickly, and a great quantity are in hospital with the Corporal Forbes, which carries them away before they have time to die.
1836		United Service Mag. (London) 327: [T]he heat during the day reflected from the rock was scarcely supportable, and was no doubt the reason why we were so soon visited by the cholera (facetiously called in the army Corporal Forbes).
1886		Yule & Burnell Hobson-Jobson 2001/1: ‘Corporal Forbes.’ A soldier’s grimly jesting name for Cholera Morbus.
1901		H. Compton Free Lance in a Far Land 80: There were [...] many distempers, such as the spotted fever, the bloody flux, and the mort-de-chien, or cholera morbus, which the sentinels in grim jest called Corporal Forbes.
1913		H. Hervey European in India 287: Asiatic Cholera, grotesquely styled by Tommy Atkins ‘Corporal Forbes,’ a corruption, presumably, of ‘Cholera Morbus,’ the scientific name – is an epidemic very common in India.