Death opens this poem in a big way. It’s hard to think about a contagious hospital without thinking about the possibility of death. If that wasn’t enough, the landscape turns out to be dead, too. Check out the "dried weeds, standing and fallen," and the "dead, brown leaves." Ultimately, the whole world we see in this poem is "lifeless in appearance." That last word is key, though. While a disease might make you really dead, the land only "appears dead." The payoff in the poem, the heartwarming conclusion, is that this isn’t the scary kind of death, but the kind that leads to rebirth.
Williams’s poem is an allegory for spiritual rebirth. It argues that plants, animals, and humans, as parts of the natural world, go through a cycle of death and resurrection/reincarnation.