Chapter 942 has just revealed that the Ebisu people have been robbed of the ability to do anything but smile thanks to Kaido and Orochi bringing the SMILE fruits into the country.
It actually makes sense that something should be the cause of this abnormal behaviour. We have children who cry, children cannot be so strongly influenced by philosophy like an adult would, so it makes no sense why the children would naturally just laugh when they should cry. It seems to be involuntary.
As to how exactly the SMILEs affect them, if it were an airborne disease or such, then everyone should be smiling, but it seems unique to Ebisu Town.
If anything, I’d just guess the people of Ebisu is also known as a leftover town and I’d guess they probably ate the leftovers of the SMILE fruits due to hunger and this led to their inability to physically not smile or not laugh.
Back in Dressrosa, SMILE was said to be an unnatural fruit that required special methods to cultivate, not too farfetched for a fruit that is toxic. In essence, leftovers of SMILE fruit are poisonous and transfer a disease. This is just a guess.
On another related note as corollary (in the sense that SMILE are apparently fruits that transmit disease)…
Dr. Hiriluk tells Chopper an anecdotal story in the 3rd person (he’s actually speaking about himself), he goes to a countries seeking famous doctors because he has an incurable heart disease. While on this venture, he comes across the Cherry Blossom Tree (Sakura Tree), and this transformed his heart (not the literal heart) which made him change from an evil individual to a righteous one. This more or less implies Dr. Hiriluk actually went to Wano.
That said, Chopper and Dr. Hiriluk’s goals are 2 sides of the same coin. Dr. Hiriluk wanted to create a drug that cured all diseases of the heart, or one that set people’s moral compasses right while Chopper’s goal is to actual cure physical diseases and illnesses. Now Chopper has presumably reached the country that Dr. Hiriluk spoke off to him.
Chopper’s dream is to find a way to cure any disease, perhaps, the Sakura tree or plant may well be the secret to achieving his dreams just like it was the cure to change the heart of Dr. Hiriluk. Maybe curing the people suffering from the side-effects of the SMILE at Ebisu would also be cured by Chopper and will be the gateway to his dreams being achieved.
On a final note, Dr. Hiriluk being a thief, I wonder if he ended up in Wano with Moriah 23 years ago when Ryuma’s corpse and Shusui were stolen.?
*Theory by HPsyche