Nanites

Nanites are the microscopic machines courtesy of Ray Palmer during the Nanite Event, and spread across the globe at the time of the explosion. Nanites exist within every living being, bonded to their molecular structure. Normally, nanites are completely harmless. Though, when activated, they mutate the biology of their host giving them grotesque physical features and special abilities, these are called E.V.O.s.

Mutations
As previously mentioned activated nanites change the hapless individual into an often monstrous and bizarre creature with special powers manifesting as well. The series has shown how the mutations can vary. In some cases creatures mainly grow greatly in size (seen in various arthropods), and many others shift into something radically different, seen time and time again throughout the episodes. Humans almost always retain some human-like qualities, though a good number of E.V.O.s encountered so far appear mindless and bent on destroying anything they can.

Befitting the chaotic nature of mutations, nanites can also make one unstable, as seen with Van Kleiss who much periodically absorb nanites from other E.V.O.s to keep himself alive.

Deactivation
The primary reason why Rex is so important to Providence is because of his amazing power and ability to cure E.V.O.s by deactivating and extracting their nanites.

It is important to note that E.V.O.s nanites cannot be deactivated if the E.V.O. in question is incurable or resists being cured.

Communication with Nanites
There have been mentions of this being possible.

In the episode "The Architect" all the workers in the idyllic village are trying to create a tower that can communicate with nanites. The tower turns out to be a trick orchestrated by the AI ZAG-RS. It appears that the workers will still attempt to work on the idea of communication and peace for humanity despite the betrayal.

In the episode "Frostbite", when Rex is overloaded with nanites he claims to talk to (or for) the nanites.

True Purpose of Nanites
In the episode "Dark Passage" the scientist Gabriel Rylander tells Rex that nanites were created to eliminate diseases and starvation across the world. But apparently some too eager scientists intentionally let the nanites roam free while they were still incomplete.

It is also said that Rex was given the original batch of nanites, which were complete, but the ones that were released were the second batch which were unstable and that is why they lead to all the E.V.O. mutations.