MCS, EHS, MCAS, POTS and dysautonomia, ME/CFS, fibromyalgia, migraine, asthma and cough hypersensitivity, PTSD and TBI, Long COVID: they co-occur in the same people far more than chance allows. Either several diseases keep happening to the same unlucky people, or they are different expressions of one underlying susceptibility. Nobody has tested which. This is the second flagship question of the research program.
The overlaps are measured. Roughly half of fibromyalgia patients meet MCS criteria; most MCAS patients meet chemical-intolerance criteria; EHS and MCS travel together; ME/CFS cohorts are full of chemical intolerance. Each pair has its own literature and none has been studied as a set.
The shared features are also measured, separately: central sensitization, autonomic dysfunction, small-fiber findings, mast-cell activation, inflammatory signatures, sensory-processing changes. The same words appear in each condition’s papers.
Two explanations fit the data. Co-occurrence: distinct conditions with distinct causes that share risk factors and so land in the same people. Common susceptibility: one high-gain state in the systems that detect and respond to environmental input, reached by different initiators and expressed through whichever inputs a person meets — the environmental-hyperresponsiveness hypothesis.
They make different predictions. If it is one vulnerability, MCS-only, EHS-only and combined groups should share measurable sensory, autonomic and immune traits, and those traits should differ from controls. If it is co-occurrence, the groups should look different on the measures that define them. That test has never been run, because nobody has had the characterized cohorts to run it. A registry of thousands, sorted by condition and by order of onset, is how the cohorts get built.
Which conditions each participant has and in what order; whether order of onset clusters; and, in later phases, whether the groups share a measurable high-gain profile or do not.
MCS and its neighbors overlap too much to be coincidence and too little to be one thing. Whether they share a vulnerability is the question, stated so the data can answer no.