TL;DR summary for if you might want to watch the series:
Five years prior to the present, a bunch of scientists thought playing around with nanites that could manipulate DNA would be a fantastic idea. They had high hopes they'd cure diseases and other fun stuff. But before they were finished, there was an accident where the facility blew up, scattering the nanites across the world and infecting every living thing within a matter of days if not hours. Mostly they just kinda chill, hanging out, but at any point they can activate and mutate their host into a rampaging monster or a mutated person who still has their mind...or if they're Bobo, give them human intelligence. (These mutations are known as "EVOs".) One year prior to the present, Six found Rex and brought him back to Providence when he saw the kid could cure some EVOs, so it's basically a war of attrition on Six's Earth where if a more permanent way of shutting down the nanites isn't found, eventually humanity will literally mutate out of existence...assuming other mutated species don't just wipe them out.
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Five years prior to the present, a bunch of scientists thought playing around with nanites that could manipulate DNA would be a fantastic idea. They had high hopes they'd cure diseases and other fun stuff. But before they were finished, there was an accident where the facility blew up, scattering the nanites across the world and infecting every living thing within a matter of days if not hours. Mostly they just kinda chill, hanging out, but at any point they can activate and mutate their host into a rampaging monster or a mutated person who still has their mind...or if they're Bobo, give them human intelligence. (These mutations are known as "EVOs".) One year prior to the present, Six found Rex and brought him back to Providence when he saw the kid could cure some EVOs, so it's basically a war of attrition on Six's Earth where if a more permanent way of shutting down the nanites isn't found, eventually humanity will literally mutate out of existence...assuming other mutated species don't just wipe them out.
Fun stuff.