system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Do hand sanitizers really kill 99.99% of germs? How can they prove that's true?
gpt
ELI5 answer: they use the product on a surface covered in bacteria, fewer than 1 in 10,000 cells remain. Therefore they can say it's 99.99% effective. Simple as that.

Longer answer:

First it's important to note that "kills 99.99% of bacteria" doesn't mean that it kills all of 99.99% of known bacterial species, or anything like that, it's literally just number of cells present on a surface. So it doesn't say anything specific about the type of bacteria that it is good against - it's not that they know of one particular species that doesn't die but the others all do. These hand sanitizers are broad in their action and don't have much in the way of specific targets against specific things like an antibiotic does. Their active ingredients are various types of alcohol... which just generally kills stuff by denaturing proteins.

The 99.99% is just due to the methodology of testing these products. They're saying that after the treatment, fewer than 1 cell in 10,000 remain - that's pretty good!

If you wanted 100% effectiveness and all bacteria dead, you could try sticking your hand in bleach, or a flamethrower... but neither of those are going to do your hand much good. Hence the alcohol-based santizer is a good compromise between effectiveness and not damaging you.