I never got sick (except one case of high fever at 3 and a case of chicken pox during elementary school) when I was younger (and more autistic). Dunno if that's pure chance or if there's a connection?
During later teenage years, I was sick constantly. I attribute this to the stress I had to cope with (and failed to cope with).
Nowadays, I get sick less and less often. I have less stress, yeah! I haven't been sick since March!
I have one allergy since childhood and recently may have developed hay fever (yet unsure) though.
I do think allergies are quite common among the autistic spectrum. But I wouldn't say that's like real sickness or that it has a connection with catching many colds/flues/other. (?)
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Autism + ADHD
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The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it. Terry Pratchett