It goes one way, or it goes another.
Bacteria are being used to produce insulin after a long time of taking insulin from animals, mainly pigs. In short, the bacteria have some DNA that makes them produce insulin, so they produce insulin, and that insulin gets collected and used to help those suffering from diabetes.
In 2203 CE, an even cooler use is figured out. If you can get bacteria to produce insulin, why not get it to produce dopamine? None of that nonsense of using other substances that then cause dopamine release, just straight up dopamine. What of other substances like Oxycontin?
Someone came up with this and figured out how to make bacteria that won't get targeted by the immune system and do this, and then injected them into the heads of paying customers. Initially awesome because you can't overwork your body's ability to produce its own dopamine because the bacteria but a few problems come up later.
Thing is, the more dopamine etc you get the less of an effect later doses have on you. Essentially, you get less sensitive to dopamine the more you get, so more is needed to cause the same amount of good feels. This also isn't a problem because the bacteria multiply by mooching off of what you get from eating... except for when it is.
Because whichever happens faster, increased dopamine release or decreased sensitivity to dopamine, will decide whether, after the initial few weeks of being generally quite happy as the bacteria grow in number, you become so overly happy all the time that you end up being completely dysfunctional as a person, or if you become so utterly miserable all the time that you end up being completely dysfunctional as a person...
This is a weird gamble, utter heaven or utter hell for the rest of your life, except you're also gambling your life. Being so overjoyed that you can't function or miserable you can't function means you can't eat, sleep or drink properly as in one state you forget to or are too shakey with joy to do it functionally and in the other you wish you were dead anyway. Essentially, you die of dehydration, and the fact that the bacteria will have significantly grown in this time means they only speed up this process with their demands.
If you're lucky, the bacteria will die before you do, and then you might not be so miserable or overjoyed and be able to grab yourself some water, followed by a meal. Most don't get to this point, but if you're at this point yourself there will almost certainly still be a few bacteria in you, regrowing in number now that there's more resources to feed off of. Thus will begin an endless cycle, at best reduced by getting an injection of nanobots that specifically target and kill these bacteria, of periods of being functional followed by being so miserable or happy that one won't want to function. Eventually, there will be one where you don't get that water or the meal, and it will be your death.
Transmission & Vectors
Injection. Some cases of it passes through blood transfusion and kissing have been noted, but these start out with far less bacteria and so at the point of noticing that you're way too happy for someone who works a 14 hour day and whose girlfriend left them for a sex bot with predetermined responses to every sentence said to it it's easier to go to a doctor, check what's up, get the bacteria removed and return to misery.
Treatment
The immune system does jack diddly squat about it because the bacteria pretend to be mitochondria, so nanobots with lasers and antibiotics, but let's be honest we're all picking nanobots with lasers, are the only way.
Cultural Reception
Very mixed, with those in support of it normally dying from its effects. The CHG ultimately decided to class it as a Type C good, meaning only the government is allowed to produce and sell it, although many megacorps illegally have tonnes of the bacteria in agar and use their dopamine to put into their employees for improved work ethic.
Type
Bacterial
Origin
Engineered
Cycle
Chronic, Acquired
Rarity
Rare
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