I got quite good answers. thanks to everyone for participating.
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You need strong alcohol to kill bacteria efficiently.
When you mix alcohol with another liquid it becomes less strong.
Whenever you put alcohol inside the body it stats to mix with the blood and other liquids inside your body and start to spread out and get thinner. So you end up with a not-very-strong alcohol that’s spread out in the body instead of the small concentrations of strong alcohol you would need to kill the bacteria.
Doctors *do* use alcohol a lot for cleaning their tools and for cleaning external wounds, cuts etc, because then they don’t have the same problem.
Because it’s good at killing *everything* when it’s concentrated enough.
Killing viruses/bacteria generally isn’t all that hard. Applying enough heat or alcohol is simple enough.
The trick is killing the virus/bacteria *without excessive harm to the host.* The levels of heat and concentrations of alcohol sufficient to kill viruses and bacteria are also generally sufficient to kill animals like humans.
That’s why we have antibiotics. They’ll kill the bacteria without killing the human.
Hand sanitiser is high alcohol. So I take the hand gel, rub it over my hands, it comes into contact with the bacteria, and it kills them.
Bacteria in my body though, I can’t scrub the insides of all my tissue with alcohol. If I could, I’d damage a whole bunch of my good cells along with killing the bacteria or viruses. I can’t wipe down every part of my insides that’s come into contact with it.
It’s a little bit like asking why I don’t drink bleach to cure a cold. The bleach would kill me first. But for cleaning things outside my body, bleach is great.
Alcohol kills cells indiscriminately, like bleach or soap. If you take it at disinfectant concentrations, it’s pure poison. That’s not an issue when you apply it to clean non-living surfaces, so that’s what it’s useful for.
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Lots of things are effective at disinfection, it’s not particularly hard to kill microorganisms. The problem is, most such substances don’t really distinguish between species, alcohol is toxic to human cells same as it’s toxic to salmonella. Not very useful to kill an infection, if you also kill the host.
What you need is something more selective, something that would target bacteria, insects or fungi, but not damage mammals much. These substances are antibiotics, pesticides, fungicides etc. Very useful in medicine, but it’s difficult to find good ones that are effective, easy to produce etc.
Viruses are even more problematic because they do not reproduce independently, they hijack your cells and have your own cells do all the work, so what are you going to do, kill your own cells?
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Alcohol is, in fact, used a the only source of medication.
Source : my (presumably) immortal grand ma
Something I’d like to add because I just learned this recently myself, [70% alcohol is a better disinfectant than 91% alcohol.](https://www.cdc.gov/infectioncontrol/guidelines/disinfection/disinfection-methods/chemical.html) Though the link provided doesn’t really explain why, the way I understand it is the higher water content of the lower percentage gives a chance for the alcohol to be absorbed into the cell walls of bacteria to destroy it. The higher percentage basically hits it so hard that it reacts by stopping fluid intake through the cell walls.
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