BILLIONAIRE Microsoft co-founder and Seattle sports mogul Paul Allen died from septic shock, a condition that can be agonisingly painful, according to a report.

A copy of the death certificate for Allen, 65, obtained by TMZ, indicated that septic shock caused him to die.

Allen died Monday in Seattle, just three weeks after announcing the return of his non-Hodgkins lymphoma, a type of cancer that had been in remission for about a decade, reports the New York Post.

Septic shock is notoriously agonising and includes symptoms such as “severe muscle pain and general discomfort,” according to Medical News Today.

It’s brought on when chemicals naturally produced by the body to fight an infection backfire and seep into the bloodstream, leading to severe inflammation. A person’s blood pressure becomes so low during septic shock that it does not respond to fluid replacement.

Allen, who owned the Seattle Seahawks and Portland Trailblazers, co-founded Microsoft in 1975. He was reportedly worth $US20 billion ($A28 billion) when he died. He was never married and had no children.

Tech CEOs mourned his loss on social media.

“Our industry has lost a pioneer and our world has lost a force for good,” Apple CEO Tim Cook tweeted.

Amazon’s Jeff Bezos lauded Allen for “his passion for invention” and tweeted that “he was relentless to the end.”