Green’s Dictionary of Slang

diarrhoea or any form of stomach upset contracted on a foreign holiday.

1957		Redlands Dly Facts (CA) 10 July 4/3: An occasional attack of ‘la tourista’.
1958		Morn. Call (Allentown, PA) 28 Feb. 38/6: My son quickly recovered from the attack of Turista-stomach which had laid him low.
1960		Wash. Post and Times Herald 10 Apr. L6: The disorder occurs often in Mexico and has been nicknamed Montezuma’s revenge, turista, and ‘the trots.’.
1972		Time 22 May 104: South of the border, it is turista or ‘the Aztec two-step’. In Asia, visitors from the West call it ‘Delhi belly.’.
1984–5		Maledicta VIII 99: The Mexican term [for diarrhoea] is las turistas (adopted into English as the touristas).