The Wave
No it's not apocalyptic, even if awful for the economy.
Natural selection has shown disease a lot of tough love before and during the CHG thanks to things like modern medicine, decontamination procedures and diseases not having an easy time jumping between planets and then proceeded to fling tonnes of food at it in the form of The Population Program and some of its consequences and the use of The Infinity Base to build planets. This gave rise to The Wave.
But what is the wave? It's descended from Chlamydia but changed a fair bit. It can survive fairly extreme hot and cold and the white blood cells of those infected will totally ignore the bacteria at the same time as being extremely infectious as one only needs to breathe out to shoot a tonne of cells into the air and one only needs to breathe in to get infected. Oh and it spreads the old way as well.
However, it doesn't do jack-diddly-squat to anyone infected with it - normally, at least.
There's this thing called mutation and it can make harmless diseases like The Wave harmful - this is especially likely when the disease is evolved from one that used to be harmful. The Wave is very good at mass-propagating within the body, at the same time as spreading between massive numbers of people at a time and this makes plenty of room for mutation to harmfulness.
This is where that hold/cold/ignored by white blood cells thing kicks in as the body's other defenses get rolling and SHAzAM you've got someone who's ill and their "natural immunity" ain't gon' do jack-diddly-squat. This is brilliant for maxing out on economic damage because there's no free healthcare in the CHG unless you or a close relative is in the military (and they're very picky about who they let in) so most people will try to walk it off, even still going to work (which potentially infects more people with the harmful strain) until they're finally pushed to the point of seeking medical help.
Now Patient 0 has been treated but that harmful strain has spread EVERYWHERE on the planet but there's still hope: nobody's allowed off of the planet if they're ill. Eventually, everyone's been treated - possibly with CHG intervention (when it comes with more economic pay-off they will just give out medical services for free on a temporary basis) - and the harmful strain is gone, sometimes with the normal strain disappearing too... until the planet is opened up again and Vanilla Wave roles in again. It's usually only a matter of months before a harmful mutation kicks up again. And along the way tonnes of economic problems arise as people are unable to work/work to high quality and so both they and the companies they work for suffer, private medical services always launch their prices up whenever a harmful Wave is going around and gas mask shops make tonnes of money.
But why's it called the Wave? News outlets talking about it skapoofing around in "waves" and the hilarity of a "second wave this year" of sick people to the hospitals.
Transmission & Vectors
Breathing. Some will wear gas masks (often stylized if they've been used enough times or by snazzy enough people) whenever they get word of a harmful strain going around.
Causes
Mutations in the vanilla version of the Wave, causing it to become harmful. The odds of this rise in locations that further encourage mutation ie: densely populated, lots of snazzy chemicals in the air, someone hasn't been doing their fusion properly (radiation) and all sorts of other fun things.
Symptoms
Tiredness, blurred vision, increased mucous production, (thinks of more things I have right as I'm typing this), regular sneezing and coughing, feeling like vomiting even though you haven't eaten anything today, vomiting the moment you eat something, sweating, runny nose, weakness, thinking your name has been said when it's just someone saying a word with the same number of syllables from a distance a simultaneous need to be hugging someone but not be near anyone, the big sad, inability to perform in be- actually I think those last ones are just me.
However, the problem does come from mutation so there've been instances of coughing blood, hallucinations, being constantly high (yeah there was a case where the bacteria did a series of reactions and one of the products of that was methanphetamine) or drunk (same but with alcohol) or orgasming (moving on) or happy or sad or angry or [...], sneezing blood, nosebleeds, swelling, diarrhea and so on. That said anything in this paragraph is definitely an exception rather than a rule.
Treatment
Nano-robotniks are injected into the blood stream and just rip apart anything that isn't at least symbiotic as they move around with smol lasers while those being treated get put in cold water so prevent overheating from the fact that lasers are being fired inside of them. These nano-robotniks then pass out in urine that is collected and... reused as the robotniks are extracted from the urine while the urine itself gets to become fertilizer.
Prognosis
If one seriously doesn't get treatment for a long time, they might die. Might. However, there've been instances of people staying ill for years before finally dying... because their tiredness prevented them from noticing that a Hive Transporters was zooming towards them.
Sequela
If people get one of those versions where they're endlessly orgasming/happy/drunk/high/etc they're likely to crave the disease again. Those who massively suffered are likely to be traumatised. That's only scratching the surface when you roll on the fact that everyone around someone would also be infected and the weird stuff that could happen as a result.
Affected Groups
Everyone can get hit, including most mammalian animals. The CHG even deliberately launched some of this into a Skaters population using their human agents in The LIR or League of Intelligent Races.
Hosts & Carriers
Lots of mammalian animals carry the Wave, although they're not usually affected when it mutates to become harmful.
Prevention
Gas masks and condoms... okay now I'm thinking of it. Damn actually kinda hot.
Epidemiology
Air and jiggle-bumping.
History
The vanilla strain appears to have come to be during the 2200s, before the CHG rose to power. The common-ness of harmful mutant strains really starts in the 2500s and continues all the way to today.
Cultural Reception
Everyone's just waiting for the day a truly lethal version comes along but none have. It kinda needs a lot of mutations to reach lethalness but by the time you're hitting lethalness you're already at "oh no people are stuck at home sick"ness and SHAzAM modern medicine has wiped you off the face of the planet, so it's unlikely that one will come along in the foreseeable future and even if it does it'll be stuck on its origin planet.
Sick culture is an "arguably" (if you're some bootlicker who enjoys having to go to work/school... in which case you'd better have a cool job/great teachers or there's something wrong with you) brilliant result of this as you can call in sick and will totally be believed and nobody will bat an eye at you showing up with a snazzy-looking gas mask (and the future-tech means they don't even have to look like gas masks so long as they cover the lower half of your face... yes, there've been a tonne of Darth Vader peeps showing up to their job in accounting).
People can't even get mad when you pretend the reason you don't wanna do them is that you're paranoid about the Wave rather than them being rude and ugly because the way the disease works is well-known. The sheer ease with which one can now, un-insultingly, turn down the diggy is incredible.
Type
Bacterial
Children
Origin
Mutated
Cycle
Short-term
Rarity
Common
Affected Species
10/10
*8/10 after ratings tax. All taxed ratings are redirected for CHG purposes and the continuation of Humanity. Anyhow, have you heard of the all-new CHGEntertainment Inc-produced movie Mustard Man? So far it's only a 2/10 but we'll be increasing that later. It follows the old Chinese legend of a soldier of the People's Republic of China who took advantage of winds blowing Goner-ward at the Battle of Suez Canal, circa 2031, by flinging tonnes of mustard gas canisters into the xeno-infested streets, taking down an entire 1 Goner per 2 humans sacrificed through the courageous move! Ticket prices are currently going at 23 credits, although anyone working for a CHG-owned company or the government/military itself can get up to 100% off, depending on their position. Please keep in mind that your failure to watch this movie within the next 3 weeks will be permanently recorded on your Patriotism Record.