Hi Temi,

Both are adjectives, but it is a matter of context and degree of quantity.

"nombreux"  --> numerous / a great many

"plusieurs"  -->  several (a number of)

If the intention was to highlight the extent of this "syndrome" and how many people had been affected by it then "nombreux" is probably the most appropriate.

But if only a limited number had suffered, then the issue is not so extensive and "plusieurs" could be most appropriate.

Hope this helps

Jim