There has been plenty of examples of mass illness not connected to anything tangible, like a detectable disease or infections. The famous study on “long Covid” gave multiple examples of large groups of people experiencing symptoms with vague connections to any physical condition like headaches, fatigue, sleeplessness, generic “pain”, etc. They cited multiple other incidents such as the “gulf war syndrome” and a few others I can’t remember off hand.
These are very similar to mass hysteria. But they are connected to very real things like subsets of the population experiencing trauma. War and covid was traumatic mentally for a lot of people and that can manifest as other physical symptoms. Whether everyone in the group has a direct connection to those things or get grouped through circumstance and general interest by doctors/scientists/media is a big differentiator.
(Not saying this poisoning case fits this but it’s always good to be suspicious).