This one reads more to me like an adventure hook than railroady.
I'd say it depends on whether this is:
(a) the premise you start off the campaign with
(b) a new adventure hook following up after a major resolved plot (e.g. "we just stopped Dagnast McBadguy")
or
(c) inflicted upon the players in order to
ensure that they do, in fact, go to where they're "supposed" to go
A & B are fine. Good, even. The first is simply expecting that players be engaged with the game you've offered to run. The second is a natural starting premise for a new adventure; not necessarily one the players
explicitly signed up for, but they kinda get the idea that yes, whenever they've completed some major objective, there will need to be a new adventure, and thus a new hook, even if this one is
slightly heavy-handed.
C, on the other hand, is not okay. It is (effectively) saying, "Unless you play the game the way I want you to, I'll take everything away." It's a crappy, petty way to control player behavior.
And this illustrates an important point:
context matters. It's incredibly important to know WHY and HOW something is happening. There's sort of a sliding scale between "perfectly reasonable no-problems DMing choices" on one end and "blatant bullcrap" on the other, and this specific thing, inflicting a disease on the party out of the blue? Yeah that falls in the grey-est of grey areas.