Contagious spreading of Ailments is getting a toggle switch in a future Sims 4 Update.
The Sims Team has made an announcement late last night about a new The Sims 4 update currently in preparations. According to Maxis, the patch will incorporate a new toggle switch in the Pack Settings, letting you reduce the contagious spreading of Ailments from The Sims 4 Enchanted by Nature.
Here’s what they’ve shared in the latest message:

We’ve heard your feedback on Ailment tuning in Enchanted by Nature and we’re developing a toggle to enable or disable Ailments spreading contagiously, coming in a future patch. Once disabled, Sims who already have Ailments will need to wait it out or seek a cure, but no more Sims will contract Ailments through spontaneous spread. By disabling both Enable Balance & Imbalance and Enable Ailment Spread, this will disable all natural sources of Ailments.
We’ll have more info to share with you once we’ve confirmed a release date.
FYI: There’s another Sims 4 Update expected to arrive on August 19th. The Sims Team describes that it will bring “new skin customization options”. We have to wait and see if this Ailments toggle switch will be added before the Aug 19th update. Until then, stay tuned for all the latest Sims 4 news on Sims Community!
Good. That option really should have been there from the start.