Morning sickness is experience by most women
Hyperemesis Gravidarium by a tiny percentage
And then within HG there is a spectrum. So, HG is fairly rare to begin with, and then the most severe form is even rarer (as in extremely).
Some common side effects of Moderate to Severe HG:
Seizures
Broken Ribs
Punctured Lungs
Torn esophagus
Aspiration Pneumonia
Acid eating the lungs (from the aspiration)
Hypoxia (blood oxygen too low)
Brain damage (from hypoxia)
Heart Attack
Miscarriage
Death
LOL... We know we don't hear of too many cases like this (although whenever one pops up, med students jump all over it... just like any other rare case), because most people are blowing it off.
Honestly, it could be normal morning sickness, or even mild HG (which can be treated with medication on an outpatient basis)... but since this is the royal line, every precaution is being taken.
HOWEVER... I just like to generally assume that whatever "it" is, is medically necessary. I'm not saying you're minimizing it, but as a high risk (bed rest) pregnancy myself (who was told over and over that I was fat and lazy for not exercising -on bedrest- by well intentioned or high horsed idiots... it makes my blood boil to see and hear people sneering at a pregnant woman following doctor's orders.