Humanity, an exceptional species that has bent the world to its will through technology and stubbornness. Sadly, they had forgotten that they were just like any other living creature: nothing but the result of DNA, which in turn is just atoms and molecules interacting in a way that's thermodynamically beneficial for their stability and existence.
Approximately 50 years ago, humanity got reminded of its place by nature itself. You see, in the deepest reaches of the wilderness, there was a virus. A virus that, over the course of centuries, slowly developed an interesting side-effect while trying to perfect its infectivity towards its main hosts: flies. So, when humanity encountered these jungle-flies due to increasing deforestation in the Amazon... they also came into contact with this new virus.
For a while, nothing seemed wrong, they didn't even notice the virus infected and spread throughout the human population, because it didn't cause illness. So why was this a problem? Well, the virus's side-effect targeted a specific gene present all of humanity. However, there was a slight variation in the gene... 10% had one, 90% had the other.
The side-effect was simple: For the variation that 10% held, it shrank the human by a factor of 2000. For the other 90%... it augmented the 5 main senses... The irony was not lost on them... and all the shrunken ones realised it was a good thing.. for otherwise they would have been truly screwed.
The virus swept rapidly throughout the world. The effects took a few weeks to develop... but soon enough everyone had become infected. And while the virus itself would eventually be beaten by the immune system... the effects were permanent.
Naturally, this changed society as a whole. New laws were made to protect those shrunken by the virus. But as time went on, they became more lenient. Tinies were often cast away from their old homes and lives by people who simply didn't want to deal with them. Government agencies existed to keep them integrated in society, but they tended to group together and form their own communities... forming cities inside cities, districts within districts, etc. Of course... there are always those with foul intentions.. and this new subclass called "micros" is an easy target...
After 5 years or so, the epidemic stopped, most of the world having been infected at some point and developed immunity to the virus. Ever since then, it occasionally returns for small local outbreaks... hitting the new generations. 30ish years ago, it became possible for parents to get intel on which variation of the aptly-dubbed "shrinking gene" their newborns held. It was a free choice... though many hearts were broken because of it, for both choices that could be made.
This brings us to our actual story. The story of a 22-year old lad from a suburban town being born unlucky with the actual shrinking gene, knowing his whole life, and eventually getting infected. A lad whose parents had only dared to have a second child when their firstborn, a girl, was already 11. A lad whose parents died in a carcrash when he himself was 12... leaving him in the care of his freshly-graduated 23 year-old sister. A lad who, given his likelihood to eventually become a mere speck, did not have many friends at all. In reality, he only had one true friend beside his sister, and she would stick by him no matter what size he was.