Coming back to the OP... "It seems fit young men can fall with...

Coming back to the OP...

"It seems fit young men can fall with it too."

Yep - I'd even go so far as to say that fit young people, with their more active immune systems, are slightly *more* likely to develop it. Like many autoimmune disorders, Guillain-Barré seems to ultimately boil down to sheer bad luck.

The problem is how the immune system works. It doesn't "design" antibodies in response to infection. Rather, it maintains a large population of cells, each producing its own random antibody. These are constantly screened so that those that happen to bind to "self" proteins are killed off, but otherwise left alone. When you get an infection, some of those random antibodies will, by chance, have some affinity for part of the antigen. The body recognised their binding, and triggers the cells producing those specific antibodies to undergo a massive expansion, with some more targeted local random mutations. Those that happen to bind more strongly are selected for further expansion, and the process repeats until you have some very strongly-binding antibodies.

For the most part, it's a system that works well, but there are clearly holes. In this case, the problem arises when an antibody develops that recognises the antigen, but *also* recognises a normal molecule within your body (myelin) that isn't a part of the "anti-self" screening pathway and isn't killed off.

The good news (what there is of it) is that it's a bit like celiac disease - the autoimmune attack only continues for as long as the "real" foreign antigen is present.