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The way I see it there are 2 cases:

1. The patient uses antibiotics and has an infection.
2. The patient does not have an infection and uses the antibiotics.

For case 1, this is a legitimate use. No one seems to be arguing against this.

For case 2, if the patient is not infected, then how could using the antibiotics cause resistance? There's no selective pressure for the bacteria to evolve resistance.

What am I missing?

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When you take antibiotics you kill not only the bacteria you wanted to, but also any bacteria sensitive to the antibiotics. That is why when you take antibiotics it can cause stomach upset, because it's killing off good gut bacteria. Killing off this bacteria can prompt it to change so that it doesn't die anymore. That's why the less you have to take antibiotics the better and taking it the way it was told to take it. You can cause resistance from not taking the whole side of skipping doses. Scenario 1 is important, scenario 2 is avoidable unless the doctor said to them as prophylaxis of something. Antibiotic misuse is another cause of antibiotic resistance.

Using antibiotics can lead to resistance. Each time you take antibiotics, sensitive bacteria are killed. But resistant germs may be left to grow and multiply. They can spread to other people. They can also cause infections that certain antibiotics cannot cure. Methicillin-resistant Staphylococcus aureus (MRSA) is one example. It causes infections that are resistant to several common antibiotics.

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1: Straight-up natural selection

Bacterial population mostly commensals and symbiotes with a few pathogenic bacteria present, some resistant -> commensals and symbiotes keep pathogens in check through eating their food -> take antibiotics -> resistant bacteria, including resistant pathogens are suddenly favored -> now it becomes a real problem

2: Somewhat roundabout and downright insidious.

Infection false positive -> take antibiotics for nonexistent "infection" -> commensals and symbiotes forced to adapt (develop resistance) or die -> small chance some of them could encode antibiotic resistance in plasmids -> those plasmids might transfer and end up in the "wrong hands" (pathogenic bacteria) -> nice going "genius", you just spawned another infectious headache for everyone else.


Somewhat unrelated, even if the new plasmid doesn't happen, improper use of antibiotics can cause an infection anyway:

Infection false positive -> take antibiotics for nonexistent "infection" -> commensals and symbiotes die off -> large amounts of "food" (stuff that bacteria eat) left up for grabs -> finish antibiotic course -> bacteria repopulate -> chance of pathogenic bacteria grabbing more food first and crowding out commensals and symbiotes -> now you're actually sick

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    This is a lot more profound answer than the currently accepted one. However, I would expect you to include references (Community Guidelines) and spelling out the "=>" arrows points you have at the bottom to make it an excellent answer. That would really deepen the understanding of any future readers! – Narusan Jul 3 '17 at 16:59

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