Of American-English origin, and borrowed from Spanish turista (translating as tourist), the noun turista, also tourista, designates diarrhoea suffered by travellers, originally and especially in Mexico.
—Synonyms: Montezuma’s Revenge, Pharaoh’s Revenge and gyppy tummy.

In the sense of diarrhoea suffered by travellers, the noun turista occurred, for example, in the column Common complaints, by Dr. Tony Smith, published in The Independent on Sunday (London, England) of Sunday 23^rd June 1991 [No. 74, page 67, column 4]:

Travellers’ diarrhoea

WOKEN in the night by the insistent demand of sudden diarrhoea, the traveller recognises the symptoms of “turista”. Travellers’ diarrhoea is a distinct disease; it is not the same as food poisoning or the more serious intestinal infections such as dysentery that may be caught abroad. What distinguishes this particular type of diarrhoea is that the local inhabitants don’t suffer from it; it is a bacterial infection to which they are immune.

The earliest occurrences that I have found of the noun turista, also tourista, designating diarrhoea suffered by travellers, are as follows, in chronological order:

1-: From The Sunday Courier and Press (Evansville, Indiana, USA) of Sunday 24^th June 1956 [Vol. 18, No. 26, page 2A, column 3]:

While Traveling, Give Your Stomach a Vacation, Also
By JOHN TROAN

When you go traveling this summer, give your stomach a vacation as well as yourself.
This advice comes from Dr. John M. Rumsey, of the Rees-Sealy Clinic, in San Diego, Calif.
Dr. Rumsey points out that many travelers, especially those going to Mexico, suffer from an intestinal condition which he calls “acute touristas.” In simple words this is a king-sized bellyache.
The victim develops acute nausea, sharp abdominal pains, cramps and explosive diarrhea. This leaves him fatigued and “mildly prostrate” for a few days.

2-: From the El Paso Herald-Post (El Paso, Texas, USA) of Thursday 27^th March 1958 [Vol. 78, No. 74, page 6, column 1]:

Food Contamination Blamed for Diarrhea

By United Press
MEXICO CITY, March 27.—Dr. Gerardo Varela, noted bacteorologist [sic] who is one of a number of specialists co-operating with the Mexican Tourism Assn. to determine why so many tourists contract diarrhea in Mexico, advanced the theory today that it is due to the contamination of foods by those who handle it.
He said only in rare cases was drinking water the cause in Mexico City, where the water has a fairly low bacteria count. He said Mexican vegetables play a secondary role.
Diarrhea was at one time so common among tourists, that Mexicans named it “la turista.”

3-: From The Diarrhea of Travelers. III. Drug Prophylaxis in Mexico, by B. H. Kean, M.D., and Somerset R. Waters, B.E., published in The New England Journal of Medicine (Boston: Massachusetts Medical Society) of Thursday 9^th July 1959 [Vol. 261, No. 2, page 71, column 2]:

ACUTE diarrhea is well known to travelers recently arrived in Italy, Turkey, Egypt, India, Mexico and many other countries, where picturesque names have been applied to the syndrome—for example, “Delhi belly,” the “Turkey trot, “Gippy tummy,” “Montezuma’s revenge” and the “G.I.’s.” […]
[…]
As part of a broader study of the problem of the diarrhea of travelers an evaluation of the value of prophylactic medication was sought. Mexico was chosen as the site of the current experiment because of the large number of American tourists (over 500,000 annually), the frequency of diarrhea in visitors (the local name of the syndrome is “turista”), the excellent laboratory facilities, which permitted concurrent bacteriologic studies reported elsewhere, and the cooperation of the health, tourist and educational authorities.

4-: From A warning to travelers, by Terry Liang, published in the Los Angeles Times (Los Angeles, California, USA) of Sunday 2^nd August 1959 [Vol. 78, Home section: page 34, column 3]:

In Mexico they call it “turista,” in India it’s “Delhi belly,” in Hong Kong it’s “running stomach.” Certainly you’ve heard them all and more. But whatever you call it, you don’t need to get travelers’ stomach. Just be careful what you eat and drink. Vacations should be fun and not a moment should be lost.

5-: From Your Health. From the Medical Society of the State of Pennsylvania, published in several Pennsylvanian newspapers on Thursday 6^th August 1959—for example in the Evening Herald (Shenandoah, Pennsylvania, USA) [89^th Year, No. 184, page 4, column 6]:

In Mexico it is called Montezuma’s Revenge, in India they call it Delhi Belly, and in Ankara it is known as the Turkey Trot.
Travelers encounter this ailment in Italy and Egypt and other warm countries where food and water and sanitation are not always of the purest quality.
Acute diarrhea is well known to travelers in all of these countries.
Last year a study was made of more than 1000 residents of the United States returning by air from Mexico.
Approximately one-third of these visitors to Mexico spending seven days or more, developed diarrhea.
Because of the frequency and fear of diarrhea, many travelers seek medical advice or resort to self-medication in an effort to protect themselves.
A total of more than 500,000 American tourists visit Mexico every year.
The frequency of diarrhea among visitors has prompted the natives to call it turista.