[RayG (OP)]

If an individual gets treated for an infection and is given antibiotics to get rid of it, how then does that drug become resistant? We know that overuse CAN cause resistance, but HOW does that process work? If each individual is a separate entity, how does the bug learn to be resistant?
I realize, many times that not all the bug is killed by the drug and that the bodies own immune system can finish the job, but can the bugs communicate between individuals or something?

[evan_au]

how then does that drug become resistant?

Unfortunately, the common abbreviation "Drug Resistance" is misleading: It sounds like the drug has become resistant.
A more accurate (but verbose) description is "Drug Resistant Pathogens", as it is the Pathogens (eg bacteria) which become resistant to the Drug (eg antibiotics).
...but the question avoids this common pitfall...

how does the bug learn to be resistant

Most of our antibiotics are derived from nature, eg from fungi, which are involved in endless chemical warfare with bacteria.
That means that in nature, there are already bacteria which are dealing with these antibiotic chemicals, and are already resistant to them.
Much of the antibiotics we take (or feed to our cows or chickens) goes straight through, meaning that bacteria in our streams and soil are exposed to increased levels of these antibiotics, encouraging the spread of resistance to this new antibiotic in the environment.

can the bugs communicate between individuals

Yes, bacteria often package up useful bits of DNA, and pass them on to others, where they can be incorporated into the genome of other bacteria, transferring antibiotic resistant genes from (say) a bacteria that lives in a stream into a bacterium that lives in the human gut or human skin.

https://en.wikipedia.org/wiki/Horizontal_gene_transfer

[alancalverd]

The simplest explanation is Darwinian.

In any population of bugs there will be some degree of variation between individuals. Given the very rapid reproduction rate of bacteria, the spread of variations within a given species at any moment is likely to be far greater than the variance, say, of humans or mice.  But let's just consider two variants A and B in equal numbers.

All the bugs in a given environment are competing for food. If A is more susceptible to antibiotic X than B, the population of A will decrease more rapidly than B when exposed to it. Thus B will dominate in that environment, having more access to food, and the next generation (two minutes later) will have a majority of B, even if only a few survive. Repeat the process thousands of times, allowing all sorts of random mutations, and at each iteration, kill off those most susceptible to X. Eventually you will have a population D, E, F or whatever, with characteristics (including infectivity and symptoms) of the A family but relatively unaffected by X.

The consequence is that you shouldn't try to minimise the dose of antibiotics in a symptomatic case: overkill is essential to prevent any B's escaping (always take all the pills,even if you feel better - it says so on the box!). Nor should you use small  prophylactic doses where there are no symptoms or probable infection - this simply warns the enemy and tilts the population balance in favor of Bs before they invade..

[vhfpmr]

how does the bug learn to be resistant?

By evolution, the three foundation stones of which are mutation, selection and heritability.

Firstly, organisms inherit their genes from their parents, this means that offspring share some of their parent's traits.
Secondly, the process of copying genes down the generations sometimes causes errors known as mutations, this means that completely new genes and traits are occasionally created.
Lastly, the environment that organisms live in is selective, so that some traits are better able to survive than others, and because of heritability, genes that survive in greater numbers also get passed on to the next generation in greater numbers. Genes that are good at surviving proliferate at the expense of those that aren't, so that after relatively few generations, a population consists of mainly, or exclusively survivors, even if the difference in the ability to survive is only very small.

That's how all life on earth evolved, but now consider a species of bacteria living in an environment containing antibiotics. Most of the bacteria will be killed, because that's what the drug is designed to do, but just occasionally there will be a mutation that confers a degree of immunity, and it will survive. Here it helps if someone has obliged by not taking the full prescribed dose of their meds, because a mutation now only needs a smaller degree of immunity. A marginal dose is good at sorting out the difference between mutations with a lot of immunity from those with less. Once a mutation is created with immunity it will then replicate, pass it's genes on to the next generation, and eventually they will produce their own mutations, some of whom will have even more immunity. These immune bugs not only survive, but in a drug-contaminated environment they thrive, because the antibiotic is doing them a favour by removing most of their competitors, and before you know it you have a population of drug resistant bacteria.

[evan_au]

The experience with antibiotics is that resistant infections are recorded the first year the antibiotic enters general use.
- And these resistant strains just become more common over time
- The manufacturer can tweak the chemical structure, which buys a bit of time

That's why many pharmaceutical companies are avoiding antibiotics - it costs billions to develop and test in clinical trials, and if it works, people only take it for a week or two and then stop. Unfortunately, it has a limited lifetime as it becomes less effective after a while.
- It's much more profitable to develop something for a chronic condition like high blood pressure or high cholesterol, as someone may take that pill every day for 20 or 30 years.