The latest victims, Kari Bowerman, 27, and Cathy Huynh, 26, were backpacking in Vietnam while on break from their jobs teaching English in South Korea.
On July 30, the friends were admitted to Khanh Hoa General Hospital in Nha Trang.
Both were vomiting, had difficulty breathing and showed signs of severe dehydration.
Huynh was eventually released from the hospital.
She returned later that night to that her friend had gone into respiratory failure and died.
Two days later, Huynh was also dead.
The travelers’ stories are just the latest in a string of mysterious tourist deaths in Southeast Asia.
Investigators with the World Health Organization suspect poisoning is to blame, but determining the origin has proven difficult.
Meanwhile, friends and family are desperate for answers.
“It’s been a nightmare trying to get information,” Bowerman’s sister Jennifer Jaques said. “No hospital reports. No police report. No nothing. Whatever happened to her we need to make sure doesn’t happen to somebody else.”
Almost immediately, international media reports began linking the deaths to an incident in Thailand in June in which two Canadian sisters died.
The bodies of Noemi Belanger, 25, and Audrey Belanger, 20, were reportedly found covered in large quantities of vomit and skin lesions, authorities said.
The pair checked into the Phi Phi Palms Residence hotel on Tuesday and were seen coming in and out later that night, the Phunket Gazette reported.
They were not seen on Wednesday and no one in their room answered a maid’s knock on Thursday.
The women, from Quebec, were found dead by a maid on Friday.
Thai newspapers have reported that eating blowfish or poisonous mushrooms could have caused the deaths, but police say the cause of death is still unknown.
Authorities found a large quantity of vomit, evidence of bleeding and that the womens’ nails had turned black, Dr. Pornthep Siriwanarangsan, director general of Disease Control Department, told Bangkok newspaper The Nation.
In February 2011, New Zealand resident Sarah Carter, 23, died in Chiang Mai, Thailand, after arriving at a local hospital with low blood pressure, difficulty breathing and dehydration from vomiting, according to the New Zealand television network TV3.
In the Downtown Inn where Carter had stayed, the Bangkok Post says three other visitors — a Thai tour guide and an elderly British couple — died between January and May 2011.
Other media reports linked Bowerman’s and Huynh’s deaths to the 2009 deaths of tourists Jill St. Onge and Julie Bergheim.
They died of similar symptoms in adjacent rooms over the same weekend at the Laleena Guesthouse on the island of Phi Phi. (The hotel has since changed its name).
Speculation on the cause arose with each death — ranging from alcohol poisoning to something the victims ate.
Evidence for the insecticide theory is mounting.
Thai police recently announced they found traces of the insect repellent DEET in the Belanger sisters’ bodies, according CBC.
Investigators believe the DEET was added as an ingredient to a popular cocktail served on the island.
The Downtown Inn was torn down this summer after the Thailand Disease Control Department concluded three of the deaths were “probably connected to the use of pesticides,” according to the Bangkok Post.
Vietnamese authorities have released very little information about the cause of death for Bowerman and Huynh.