Timothy Bingham, 28, was first badly affected by the flu at the age of two but was able to largely recover.

A second bout at age four badly affected his mobility and the dexterity in his arms but he was able to achieve 80% recovery.

At the age of six, Mr Bingham became fully paralysed under similar circumstances. He has used a wheelchair ever since and needs constant care.

A connection was made between the infections and his reaction, and risks have been managed since.

“It’s hard for him to chew and he can’t drink unassisted,” said Tim’s mother Kate Bingham.

“He can’t move in bed so needs turning throughout the night. The things we all take for granted he can’t do.”

She added: “The discovery of a gene which is linked to what happened to Tim means everything to me.

“For so long we have lived with uncertainty of not knowing the full picture.

“This breakthrough brings us great hope as it will do to all those people who have waited years for answers.”

The study was funded by the National Institute for Health and Care Research, LifeArc and the Wellcome Trust.