Why do bathroom cleansers only kill 99.9% of germs and not all of them?

Where 0.1% of germs lurk ... the bathroom. Photograph: Johnny Green/PA

All bathroom cleansers say they can kill 99.9% of known germs. What is this one-in-1,000 germ that has a charmed existence?

Dennis Hawkins, Leominster, Herefordshire

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