Cancer is a major aspect of Annihilation's narrative, namely that Ventress is dying of cancer as she heads into the Shimmer, and the way the Shimmer alters the Earth's landscape, causing tumour-like objects to grow. Just like cancer, it is relentlessly pursuing its biological mandate without any consciousness or malevolence.
However, Garland also interweaves this with the notion of creation, as Lena plainly states that she believes the Shimmer does not desire destruction - because it doesn't desire anything, of course - but rather its "goal" is instead to change Earth. After all, its "cancerous" properties also facilitate the prismatic DNA soup which allows Josie to become a plant statue, and transforms Ventress from a slow-dying slab of meat into a beautiful, glowing, nebulous entity.
So, what is destruction and what is creation? That is at the heart of this movie: Lena destroys the "cancer" that is the Shimmer, and her original self is, for all intents and purposes, broken down and reconstituted into something new, thanks to the Shimmer's effects. This is evidenced by not only her glowing eyes, but also the water trail visible on Lena's glass at the end of the movie, separating into two trails just as a cell divides in two.
It's an extremely interesting way to consider the nature of the human body, its inevitable breaking down, transformation and destruction. But Annihilation also has much to say about the prospect of immortality...
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