Grieving family of an American couple who mysteriously died while holidaying in Fiji have revealed the contents of a final text, and how they are preparing to deliver the tragic news to the couple's orphaned two-year-old son.

Michelle Paul sent her father, Marc Calanog, a text on the afternoon of May 24 saying both she and husband, David Paul, were feeling sick, the Fort Worth Star-Telegram reported.

"We are both going to doctor now. We have been throwing up for eight hours. David also has diarrhoea. My hands are numb," she wrote.

Calanog stayed in contact with the couple via text as they went to a clinic and later returned to their hotel having received, according to Michelle's texts, copious amounts of fluids and electrolyte packets.

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But on May 25 he was told his 35-year-old daughter was dead.

Two days later his 38-year-old son-in-law — who had text his father-in-law about receiving IV drips in hospital and possibly being flown to Australia for an operation — also succumbed to the mystery illness.

Texas couple Michelle and David Paul lived active, adventurous lives, but succumbed to a mystery illness days after arriving in Fiji last month. Photo / Supplied
Texas couple Michelle and David Paul lived active, adventurous lives, but succumbed to a mystery illness days after arriving in Fiji last month. Photo / Supplied

The cause of the deaths, which occurred just a few days into the adventurous couple's island escape, remained unknown.

Fiji's Ministry of Health and Medical Services said in a press release yesterday the flu had been ruled out, and that "at this stage we do not believe there is any risk to the public".

An investigation is under way and a small number of staff and health workers who came into close contact with the couple were being monitored, but remained well. The ministry was also working with Fiji Police's forensics department, the World Health Organisation, the US Centers for Disease Control and Prevention and the US Embassy to find the cause of death.

Meanwhile, the couple's bodies remain in Fiji — once a cause of death has been determined they'll be taken to the US — and family have begun the heartbreaking process of trying to understand what went wrong.

He was "stunned" when he was told of the tragedy, Calanog told the Star-Telegram.

"I almost cried, but I didn't cry because I was holding on to my emotions ... I would say I was emotionally shocked."

A funeral service for the Fort Worth couple will eventually be held in Nevada, where Calanog lives, but the family do not yet know whether the bodies will be able to returned as they are, or whether they will have to be cremated.

If there deaths were caused by an infectious disease the bodies would be cremated as they only other option was for the remains to be returned inside hermetically sealed bags, Calanog told the Star-Telegram.

David and Michelle Paul, pictured in Fiji shortly before their deaths. Photo / Supplied
David and Michelle Paul, pictured in Fiji shortly before their deaths. Photo / Supplied

In the meantime, plans are being made for the lives the couple left behind, two children and a dog.

Finding the dog, Zooey, involved a desperate search of local pet kennels because no one knew where the couple had booked her to stay. After many phone calls and a Facebook plea, Zooey was found.

The daughter the couple raised, David's from a previous marriage, would live with her mother in Hawaii, while Calanog and his wife were starting the paperwork to adopt the couple's two-year-old son, the Star-Telegram reported.

The little boy doesn't yet know his parents are dead.

Calanog planned to speak with a priest so he could figure out what to tell the toddler, he told the Star-Telegram.

"I'll have to learn how to do that."