A grieving family described their heartbreak after their adorable five-year-old son suddenly died after contracting an ultra-rare disease

Justin Vu, 5, died in early July at a children's hospital in Portland during a trip to visit family. Doctors eventually diagnosed him with idiopathic systemic capillary leak syndrome (SCLS), also known as Clarkson's disease. 

Capillary leak syndrome occurs when plasma leaks out of blood vessels, leading to a rapid drop in blood pressure and organ failure, according to the Cleveland Clinic.

There is no cure for Clarkson's, and it can be fatal. Symptoms can be managed, but since the disease is often misdiagnosed, it can go untreated. 

The disease is so rare that it has impacted fewer than 500 people worldwide. Capillary leak syndrome mainly happens in adults and is extremely rare in children, the Cleveland Clinic says. 

'We're trying to figure out what life looks like now and taking it day by day,' Justin's mother, Terese Peden, told local ABC affiliate KDVR. 

'We kind of had a mentality that we weren't leaving there without him, and so we really just fought. Us and our whole family were Team Justin, and we held out hope until that next morning when they told us there was nothing they could really do.' 

Justin's father, Viet Vu, wrote on Facebook that his son was on a trip with his sisters and mom and was enjoying the zoo and the beach before he started complaining of a stomachache. 

Just a few days after landing in Oregon from Colorado, Peden took her son to the hospital, fearing he had appendicitis. 

Doctors struggled to figure out what was causing his symptoms and admitted him to the intensive care unit, where his condition began to worsen. 

Vu rushed to Portland to see his son, but Justin had already been placed on life support. Doctors told the concerned father that they had not seen anything like it in their careers.

'My son fought as hard as he could until July 8th. By that time his organs were failing him. The Drs told us chances for recovery and survival were slim,' Vu wrote. 

'And at this point my boy wasn’t even recognizable. His face and his body were soooo swollen from the fluids they were putting in to keep him alive. To save him from a catastrophic event, the Drs advised us to decide how to let him go peacefully.'

He added that Justin was surrounded by family reading him bedtime stories when he died. 

A family friend has set up a GoFundMe for Justin's parents to support their grief and cover funeral expenses. It has raised more than $44,000 as of Thursday.  

'Justin was a sweet and loving little boy who brought joy to everyone around him. He loved Pokémon, dinosaurs, and sharks,' the fundraiser's description read. 

'He had a contagious smile, and filled every room with his laughter and endless energy.'

Justin's family is hoping to spread the word about Clarkson's so others are aware of the fatal and intense condition. 

Doctors have yet to determine the cause of systemic capillary leak syndrome. In the days leading up to an attack, patients may experience abdominal pain, fatigue, nausea, upper respiratory tract infections and increased thirst, according to the Cleveland Clinic.