Worm Story by Morris Gleitzman

Bigger and fatter than all the other microbes in the valley. Wilton feels ostracised and out of place. But even more worryingly, their world has become unhealthy and toxic with mass illness and flash storms. Setting out to find out what is wrong, Wilton meets Algy, a tiny microbe small enough to sit on his shoulder (and take refuge in his guts) who accompanies Wilton on his quest. Taking a wrong turn in an effort to escape some killer fungus, they inadvertently end up outside - and discover (thanks to some helpful bacteria) that the world in which they live is actually one giant organism called Janet.

ISBN: 9780143308935