Where did I mention poison or disease?
You're neglecting the fact that bio-weapons are HIGHLY contagious, nondiscriminatory attacks. That means, any and everybody is a victim of it. If a shinobi coughs on a human? Breaths on him? He's now infected and the human race slowly dies from their own attack. Furthermore, People like minato, tobirama and Obito, who can teleport to arbitrary locations, can effortlessly, with the help of SM users, teleport to each General and spread the infection to them. From there the human race is now being infected by their disease from the top-down chain of command(General giving orders to his men), and down-up(units on the field fighting on the front line who then infect their squad leader, who then slowly infect the next up in command).
Lastly, you're completely and utterly neglecting time. That is, bio-weapons are meant as slow killing, stealth attacks. They're not meant for quick death. Shit like ebola, small pox, anthrax, etc. take hours, days, if not months to kill someone and usually can be cured. The human race wouldn't survive near that long. 40k+ shinobi died in one night, against the juubi, and shinobi are far far far faster than humans.
SIDE NOTE: You're also neglecting to factor in these bio-attacks actually delivering successfully. Sage users -- kabuto, naruto, hashirama, jiraiya -- can sense every single thing around them (evidenced by the fact that madara could sense sasuke's steel sword). The minute the human race launches the 'delivery system' for these attacks, they'll be intercepted by a jinton, omnyoudon, shinra tensei, etc and completely reduced to absolutely useless.
tl;dr; Bio attacks and Poison affects humans too. You just helped kill the human race. Good job.
