So i am a bit lost how to adjust and decide how high i should set the DC for my disease i know a cult is kidnapping children making them sick and let people like the PCs rescue the children or they get ransom money from the parents and hoping the children are contagious and still don't show any signs of being sick and so spreading something horrible.
But they all just created their characters and i don't want to kill them right away or kill the inhabitants of Absalom Station. Because Remove Affliction is a Mystic spell only and Starfinder is using the spells know mechanic for spellcasters having at least a level 7 Mystic and this Mystic happens to know this spell as one of the 2 level 3 spells... It seems not be so widespread to prevent a pandemic.
On the other hand i want to set up a threat for not only the players but also the ingame world.
My players are all only level 1 and on the way to rescue some children with a nasty surprise after they saved the day got paid and may or not may be disesase carriers.
I am giving the disease an onset time of 3 to 5 days.
So, CR 1/3 have a... +0 save. Anything DC 13+ is probably going to spread to the station as a whole, and low level PCs have a good chance of failure. Trained doctors can give patients a +4 bonus to the next save assuming they can't figure out a cure. A 50 Cr medpatch can give anyone a +10 bonus to this check, probably enough for them to make the check.
So, depending on how fast it spreads, and how far down the track people get before they go in for a medical checkup, this will quickly deplete medical supplies, and stretch doctors thin.
All that said, the lvl 0 detect affliction is on both spell lists. Hiring casters can quickly sort out the diseased and clean, allowing a quarantine, and letting those who get over the disease out.
So a DC 15 should be okay?
There is such an easy to read entry for traps with the CR right there ready to use but nothing for diseases and poisons.
Oh yes i should let casters do that once the disease is out there and they know some disease is out there at public places.
And i see that the caster using Detect Affliction can fail the skill check to see if it is the new created disease by that cult or something that is known for ages.
You don't really need to worry about the stats of the disease in relation to the entire population of absalom station - if you don't want it to be a pandemic just don't say it is, you don't have to roll saves for every npc or try to accurately simulate the spread of the disease if it doesn't serve the narrative - you actually straight up shouldn't do those things.
Many of the diseases or other things that can spread in rpgs, if you actually simulated their spread using the rules as written, would become world ending pandemics - this is because they are written to be a threat to adventurers who have really good stats, while the average commoner has almost no chance of making their saves.
I remember reading an article in which someone used maths to simulate what would happen if a wizard in 3.5e summoned a single shadow and let it loose in a city - iirc it is essentially impossible for a civilisation (or a world) to survive a single shadow after it has had a few days to start spreading if you simulate what happens using the rules as written, as shadows are difficult to deal with and can multiply exponentially once unleashed on a population.
My point is, the rules of rpg systems are not intended to be used to simulate the entire world literally - the are supposed to only simulate how things interact with the adventuring party.
So, I would advise you to not get into the trap of worrying about the save DC being too high for off-camera npcs, as you don't need to make rolls for those npcs. Just set the dc to be the appropriate level of challenge for the party.
Considering the above and the fact that the PCs are starting out. I'd pick DC 13 or 14 if it was my current players. Simply based on their average fort save.
You want to hit one or two players, not all of them.
I just stick with the 15 and rolled for my players after they gave me their perception skills and their saves with the modifiers. And the soldier a Vesk who also bitten one infectious who was still in the stage without any symptoms is still healthy. So that Vesk is tough and i will decide if he will develop an immunity for this artificial designed disease by crazy cultists later in the game.
We agreed for surprise attacks, traps and so on i roll for them with all their modifiers should they be not aware of things as a character.
And my notes say only 2 out of 4 players are infected. I should let a few children return now but they are still chasing the kidnappers of the children they are hired to rescue.
Sadly for them the threat of being killed is not so useful for those people they will soon learn who they really are and fight against.
Don't worry i will not role saving throws for all the living beings on Absalom Station ;) i was just thinking which tools a society with magic and modern medicine has to offer in case of a station wide outbreak.
And a slow progress for this disease is part of my custom design for the first part of my campaign. Not sure if those cultist will return later or not with a different approach.