Why disinfectants kill only 99.9% of germs

How do flysprays work, and are they harmful for humans? What's "muscle memory" and where are these memories stored? Why do disinfectants "only" kill 99.9% of germs? How do I cleanse domestic rainwater tanks to kill microbes? How does freefall - as described by orbiting astronauts - work? If I let my pet cobra go, will it survive in the wild? What's the basis of sweating, and why do some people sweat more than others? Dr Chris Smith and CapeTalk's Clarence Ford have the answers... Like this podcast? Please help us by supporting the Naked Scientists