Soul Bound by Zoe Piper

Marshall Cavendish is an ex-Special Forces operative who is a shadow of his former self. For the past two years, he’s been fighting a mystery illness that the doctors have no answers for despite all possible avenues being explored.
Aaron West is the Alpha of the West Mill Pack, whose health is slowly deteriorating. When the Pack doctor tells him he’s found a human with the same symptoms, and he’s bringing him home to run tests, Aaron tries not to get his hopes up a cure will finally be found.
When Marshall and Aaron meet, there is a connection like no other – a rare, once-in-a-generation soul bond that needs to be sealed for them to regain full health. Together they must navigate an uncharted path as Marshall’s human body struggles to accept the shifter bond and whether his growing feelings for Aaron are real or a result of the soul bond.