Irons had withdrawn from an ASP World Tour event in Puerto Rico, the Rip Curl Search, over the weekend due to the illness and died "during a layover en route to his home in Kauai, Hawaii," according to an Irons family statement released by the ASP.
The surfwear company Billabong, an Irons sponsor, said Irons reportedly had been battling dengue fever, a viral disease transmitted by mosquitoes, but that could not be confirmed. The disease normally is not fatal.
The ASP's prior event, in which Irons had competed, had been held Oct. 7-19 in Portugal.
"The thoughts and sympathies of the ASP family go out to the Irons family as the entire sporting world mourns this tragedy," the ASP said. The 6-foot-tall Irons was considered "one of the best competitive surfers of all time," it said.
Irons, who grew up in Hanalei on the island of Kauai, started his career on the ASP World Tour in 1998 and went on to capture 20 victories and the three consecutive world championships from 2002 to 2004. He was a four-time winner of the prestigious Vans Triple Crown of Surfing event in Hawaii.
"The world of surfing mourns an incredibly sad loss," Billabong said a statement.
Irons was known for his pitched competitive battles with Kelly Slater, 38, who is attempting to win his 10th world championship this year.
Irons had taken last year off and, after returning this season, was having an up-and-down year. He was 16th in the point standings entering the Puerto Rico event.
Irons was expected to compete against two Australian surfers in his first heat Saturday, but he didn't show up and then withdrew Sunday, the AP reported. He also was scheduled to enter the next Vans Triple Crown of Surfing in Hawaii starting next week.
Survivors include his wife, Lyndie.
Times staff writer Thomas H. Maugh II contributed to this report.