'We have been throwing up for 8 hours': Texas couple who died in Fiji texted relatives

In text messages to their relatives, a Texas couple on a dream trip to Fiji complained of being violently ill but offered no indication they were worried the mystery ailment that afflicted them would take their lives.

Michelle and David Paul died two days apart after arriving in Fiji on May 22 and becoming gravely ill soon after.

The Fiji Health Ministry has been working with the U.S. Centers for Disease Control and Prevention and the World Health Organization to determine the malady that took the lives of two seemingly healthy people.

Text messages obtained by ABC News from relatives shed light on the couple's deteriorating conditions and the treatment they were offered by doctors at a local clinic.

"We are both going to doctor now," Michelle Paul, 35, texted her parents, who live in Nevada, soon after she and her husband arrived in Fiji. "We have been throwing up for 8 hours. Dave has diarrhea. My hands are numb. We will text when we can."

After returning to their hotel, Michelle Paul wrote another text to her mother, Juliet Calanog, giving her an update on the treatment they received at the clinic.

"We just got back from the clinic. They gave us fluids and anti-nausea drip," Michelle Paul wrote. "They gave us electrolyte packets and anti-nausea pills. We still don't feel 100%. Going to rest in our room."

Juliet Calanog responded, advising her daughter in a text to "slow down."

"Take care," she wrote. "Drink a lot of fluid, bottled water. You need to rest."

But the treatments the couple received didn't work.

Michelle Paul died on May 25 and her 37-year-old husband passed away two days later.

The Health Ministry in Fiji said an investigation into the cause of death is ongoing.

"Influenza has been ruled out, and at this stage, we do not believe there is a risk to the public," the Fiji Ministry of Health said in a statement released on Tuesday. "It would be premature to speculate further on the cause of death until the investigation is complete."

Five people, including medical staff who came in close contact with the couple at the clinic where they were treated and both died, have been quarantined and are being monitored at a hospital in Nadi, Fiji, as a precaution.

The Health Ministry said those under observation are all "currently well."

Medical staff at the Nadi Hospital told ABC News that relatives of the five people under observation are not being allowed to visit them.

A spokesperson for the World Health Organization, or WHO, said in a statement on Tuesday the agency is "providing access to laboratory and other technical services in Australia" to analyze specimens taken during autopsies of Michelle and David Paul.

"We are working closely with the Ministry, the CDC and our partners," WHO's statement reads.

A Fiji Health Ministry spokeswoman would only confirm to ABC News that blood samples from the couple were sent to a laboratory abroad for testing and the results could come back as early as the end of this week.

"I'm stuck in a nightmare," David Paul's sister, Rebecca Ward, said in an interview on ABC's "Good Morning America" on Wednesday. "You think you're going to have an answer today and you get a call that they don't know."