It was a fungus spore that finally did it, knocking humans off the top of the food chain, with a lot of help from a pharma company too busy thinking about oversized profits rather than subpar outcomes.
Most folks died, if not from the fungus, then from the riots and starvation that came when the grocery store shelves emptied and the spigots ran dry. Plenty of other folks lived on with the fungus growing in their bones, rooting in their brains, and turning them three-quarters crazy.  
That's when I moved underground and sealed the hatch shut on my backyard bunker. I was safe, with all the food, water, and entertainment I needed for five or six years. I had DVDs to watch, music to listen to, and books to read. Prepping had paid off for me.  
At least it seemed so at first. 
After a time, my radio stopped picking up signals from the world up top and for all I knew I was the last man alive on Earth. I'd planned for that possibility, yet never truly accepted things could turn out that way. The loneliness set in, gnawing at me and making me think crazy thoughts, including one that would change everything. Two years on, I opened the hatch and climbed out of my bunker.  
I had to see if anyone but me was left.