Neither a local public entity nor a public employee is liable for injury proximately caused by the failure of the employee to furnish or obtain medical care for a prisoner in his custody; but this Section shall not apply where the employee, acting within the scope of his employment, knows from his observation of conditions that the prisoner is in need of immediate medical care and, through willful and wanton conduct, fails to take reasonable action to summon medical care. Nothing in this Section requires the periodic inspection of prisoners.