The patient will be more/most 'vulnerable to' infection immediately after surgery.
He was very 'vulnerable' after his divorce.
The troops were in a 'vulnerable' position.
(..) Only then, will the Dark Lord trust you completely. It will come a time, when Harry Potter, must be told something. But you must wait, until Voldemort is at his most vulnerable. Must be told what? On the night, Lord Voldemort went to Godric's Hollow to kill Harry... And Lily Potter cast (..)2011 Harry Potter and the Deathly Hallows: Part 2