I'm not sure whether it's described as an illness as such but I have hyperhidrosis which really sucks. It makes my hands go really sweaty almost all of the time, which gets really awkward if I'm in a situation where I know I'm going of have to shake someone's hand because I get uber nervous and so they get even worse.
I had to shake a few peoples hand on my work experience and the looks on all of their faces was painful when I did, one even wiped their hand on their trousers afterwards. It probably sounds silly reading it here but when this always happens it really knocks your confidence, knowing every time you touch someone in any way you'll get that reaction. Even high fiving someone is impossible.
It can affect the rest of my body too in high temperatures so I always have to dress really light in the summer and generally stick to white/black shirts or else I look like a complete mess. The worst thing is that there's no medication that makes it better, you're stuck with it for life, which does make me concerned about the future in terms of jobs and relationships and such.