There are viruses and bacteria out there that are killing us every day. We have some form of dominion over the larger beasts, but the smaller ones are always one step ahead of us (thanks to their ability to evolve and adapt faster).
I mean, come on, we still don't have a cure for the common cold. Flu can kill millions, and frequently does every year. It took 20 years and a global effort to get rid of smallpox, and there is no guarantee that it worked. Bacteria are the most evolved and complex organisms on the planet; they've literally been around since the "beginning". 4 billion years of evolution does a lot.
I mean, come on, we still don't have a cure for the common cold. Flu can kill millions, and frequently does every year. It took 20 years and a global effort to get rid of smallpox, and there is no guarantee that it worked. Bacteria are the most evolved and complex organisms on the planet; they've literally been around since the "beginning". 4 billion years of evolution does a lot.