Not Mush-Room

With many areas of the planet becoming inhospitable due to war, humanity finds a way to downsize itself using insect DNA, abandoning the now-oversized technology of the past. The new humanity starts to recreate civilisation but soon discovers an unexpected aftermath of the war - sentient, and highly deadly, fungi. Without advanced weapons to rely on, humanity must use their new insect-like bodies to fight against this new enemy or face extinction.