Describe the background of the poem "Ode to the Nightingale."

John Keats (1795–1821) was a British Romantic poet and medical student who suffered and eventually died prematurely from tuberculosis.
The poem itself was written in the spring of 1819, when Keats was experiencing declining health but a great flowering of his poetic powers. Although he was focusing on his poetry instead of medical studies by this point, the suffering of people in hospitals was still very much in his awareness.
He had also enjoyed an idyllic walking tour with friends in 1818 and had, as most of the other Romantics, a deep emotional connection to nature, seeing it as something that could help people transcend human suffering. There is something almost religious in the joy Keats finds in the bird's song.
Another important element of the background of this poem is personal. Keats had a strong sense of his own physical frailty and reacted to this by seeing that poetry and the arts, as well as nature, were a way to participate in beauty and joy via imagination. Even if Keats in the poem was too frail to go for a physical walk, the song of the bird could give him a share of all the joy and beauty the bird itself was experiencing, via the power of imagination. Thus, readers accompany Keats on a journey of the mind that is made real and concrete by his words.

John Keats, English Romantic poet, had a remarkably short career; his greatest odes were written not long before his death at age twenty-five. Keats had lived at Hampstead, a rural area just north of London, when he wrote "Ode to a Nightingale."
Because Keats had been looking after his brother, Tom, who was sick with tuberculosis, he had been thinking about the transient nature of life.  That train of thought, coupled with the bucolic setting in which he'd lived and heard bird songs, combined to produce the conceit of the song of the nightingale, in which Keats observes a sort of immortality.  Though the bird would pass away, its beautiful song would endure in the natural world.  
Odes are addressed to a particular subject, and Keats' poem is meant to celebrate the bird whose song will remain immortal, a contrast to the lives of men.
https://poets.org/poet/john-keats