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I've had it on the back of my neck since I was 12. Could never put anything on it as the cream just made my hairline sticky and sweaty which just made everything worse. After using nasty tgel for 10 years I've discovered that the body shop's ginger anti dandruff shampoo is practically a miracle cure and apparently I'm not the only one.
That was in 1991. I finally got shot of it in 2013 when, coincidentally, I finally got my underactive thyroid treated. Up to then I had had a permanent patch on one arm and other more transient ones on my scalp, my legs, the bottoms of my feet and my bum. I am finally able to wear short sleeves again, just as my bar mitzvah wings go into overdrive!
That's not what you wanted to hear, is it?
Your chest infection is probably significant. Have a google, there seems to be lots on there. Looks like it's a strep thing.
I have it and am from a family that do. It can show itself at any age - your doctor should know that! My Grandad got it in his 80s, and I'm in my 30s and have the paint drop one that's common in children not adults.
There is a thing like others have said about having it after throat infections. Mine used to get worse after those.
In terms of managing it - trial and error really. Some people prefer the stronger creams on prescription, others find cutting out perfumed soaps etc can help. Mine gets better in the sun, but its different for everyone. Good luck x