At least 26 E. coli O157:H7 illnesses in northeastern Canada have now been linked to California-grown lettuce served at KFC and Taco Bell outlets, health officials say.

Dr. Frank Atherton, Nova Scotia's deputy chief medical officer, on Friday said health officials believe that is the "common source across three provinces" of the outbreak that sickened people beginning in late December.

The E. coli outbreak involves ten confirmed cases in Nova Scotia, six in neighboring New Brunswick, and ten more in Ontario. While the source of the contamination has been found, the cause is another story.

Atherton says some of the KFC/Taco Bell outlets have been inspected, as has their Ontario-based supplier, and no problems have been found. Federal inspectors have been called in to continue the investigation by moving further up the food chain.

San Francisco-based FreshPoint Inc. distributed the California grown lettuce to KFC and Taco Bell in Canada. The grower of the lettuce has not yet been named.

Last year, an E. coli outbreak in New Brunswick was eventually traced to lettuce grown in southern California.

Canadian health officials do not expect to see more cases in the current outbreak because the lettuce involved is past its shelf life and in all likelihood is no longer being served.

One of the 26 patients, a woman in Nova Scotia, is still hospitalized.

Parent company of KFC and Taco Bell is Yum! Brands Inc., which has over 38,000 restaurants in 120 countries around the world. Its other brands include Pizza Hut, WingStreet, Long John Silver's and A&W.

Leafy greens have been the source of foodborne illnesses before Canada. Here are some examples from the files of the FDA and Outbreak Database.

August 2012: At least one sickened by E. coli O157:H7-tainted romaine lettuce from Tanimura and Antle from California.

April 2012: 18 ill with E. coli O157:H7 from romaine lettuce in Canada and California from Amazing Coachella out of California.

October 2009: 4 sick with E. coli O157:H7 from lettuce served at a Wendy's in Ontario.

July 2009: 12 Canadians sickened with Salmonella on Taylor Farms lettuce.

October 2008: E. coli O157:H7 on romaine lettuce in Ontario at three restaurants:

1. Johnathan's Family Restaurant

2. Little Red Rooster

3. M.T. Bellies

August 2008: Aunt Mid's iceberg lettuce sickens 3 with E. coli O157:H7

In 2006, lettuce served at Taco Bell restaurants sickened 78 people with E. coli O157:H7 on the east coast of the United States.

KFC restaurants in Ohio were linked to 18 E. coli O157:H7 cases in 1999.