What happens when you fall sick on a cruise?

To a lot of people booking a cruise may not be an annual holiday, it may be a once in a life time experience where it's important that every single aspect is perfect. To be honest even if it's not a once in a life time experience, even if you're the kind of person that likes to cruise three of four times a year, it's still nice to know that everything will go according to plan.
But what happens when things don't? What happens when you fall ill on a cruise ship?