Describing her own illness in "Fever 103 Degrees," Sylvia Plath wrote:

Darling, all night
I have been flickering, off, on, off, on.
The sheets grow heavy as a lecher's kiss. 

Plath's poem unfolds in the wearied blur of fever, and indeed many poets have written vivid depictions of their own bouts with illness. Some poems describe the physical sensations of various sicknesses and injuries, and some use the physical ailment as a departure to wonder at larger questions. John Milton’s "When I Consider How My Light is Spent," for example, is a poem about the onset of blindness, but also about what this loss of sight signifies about the poet's relationship to God.

Poems of illness can also be about the illness of others, and these poems, usually written upon the passing of a loved one, a family member, or an admired figure, are often written as elegies. Consider, for example, W.H. Auden’s remarkable "In Memory of W. B. Yeats," where, in the second stanza, Auden zeroes in on the last afternoon of Yeats’s life. The lines refuse sentimentality while maintaining a respectful imaginative distance.

But for him it was his last afternoon as himself,
An afternoon of nurses and rumours;
The provinces of his body revolted,
The squares of his mind were empty,
Silence invaded the suburbs,
The current of his feeling failed; he became his admirers. 

Jane Kenyon’s poem "Having it Out With Melancholy" is a poem written in nine sections, the first eight of which deal with depression, its constant shadow. The final section, however, offers this fleeting glimpse of the promise of something more, just beyond the reach of depression’s clutches:

What hurt me so terribly 
all my life until this moment? 
How I love the small, swiftly 
beating heart of the bird 
singing in the great maples; 
its bright, unequivocal eye. 

There is also a strong tradition of poems in memory of loved ones and family, as in Hayden Carruth’s long, wrenching poem, "Dearest M--", written after his daughter’s protracted battle and death from cancer. Or Donald Hall’s poems written in memory of his wife, the poet Jane Kenyon, who died of leukemia in 1995. In "Her Long Illness," Hall recounts the months-long endgame of Kenyon’s sickness and the distraction of the mundane. This poem can perhaps server as a field guide for those in similar situations:

    Daybreak until night fall,
he sat by his wife at the hospital
    while chemotherapy dripped
through the catheter into her heart.
    He drank coffee and read
the Globe. He paced; he worked
    on poems; he rubbed her back
and read aloud. Overcome with dread,
    they wept and affirmed
their love for each other, witlessly,
    over and over again. 

Poems of illness seem to cover a wide range of poetic forms: elegy, valediction, remembrance, ode, instruction, love. Here is a small but bountiful selection of poems that span the wide range of poetic responses to illness.

"In Memory of W. B. Yeats" by W.H. Auden
"Visit to St. Elizabeths" by Elizabeth Bishop
"Faustina, or Rock Roses" by Elizabeth Bishop
"Dearest M--" by Hayden Carruth
"An Anatomy of Migraine" by Amy Clampitt
"Mastectomy" by Wanda Coleman
"This Summer" by Liam Rector
"Hymn to God, My God, in my Sickness" by John Donne
"As One does Sickness over (957)" by Emily Dickinson
"Her Long Illness" by Donald Hall
"Atlantis" by Mark Doty
"How Some of It Happened" by Marie Howe
"The Subalterns" by Thomas Hardy
"A Story About the Body" by Robert Hass
"Having it Out with Melancholy" by Jane Kenyon
"Try Anyway" by Noelle Kocot
"When I Consider How My Light is Spent" by John Milton
"Fever 103 Degrees" by Sylvia Plath
"Maison Cartier" by Robyn Schiff

Finally, an excellent anthology on this subject is Articulations: The Body and Illness in Poetry, edited by Jon Mukand and published in 1994 by the University of Iowa Press.