FORT WORTH, Texas -- A Fort Worth, Texas, couple has died after contracting a mysterious illness while on vacation.Michelle and David Paul married a few years ago. Two weeks ago the couple left behind David's daughter from a previous relationship, their 2-year-old son and their dog Zooey for the trip of a lifetime to Fiji - not knowing they'd be leaving their young family behind permanently."Anything can happen," said Tracey Calanog, sister-in-law to the couple through her husband, who is Michelle's brother.Calanog said relatives got the call the day before the couple was scheduled to return home to Fort Worth."They knew something was going on in their body and were able to get to the hospital," she told KVUE The family says the early indication is some type of virus, strong enough to debilitate two young, healthy people.They say Michelle died first, then David, an Air Force veteran, before he could be airlifted to Australia for treatment. Their exact cause of death is still a mystery.KVUE spoke to the U.S. Embassy in Fiji, which declined to comment on the Pauls' case specifically. The U.S. Embassy did say, however, that the Fijian government had been warning about a dangerous flu season hitting the southern hemisphere.The couple's children were left with family, but their dog's whereabouts were unknown - the Pauls had boarded her but didn't tell anyone where she had been boarded."We were literally just calling places on a hope and a prayer," Calanog said.A community of animal lovers in the area helped track Zooey down and get her back in her grieving family's arms."We know that Michelle and David are with us in Zooey," Calanog said.