You can be sure for every woman admitted there is another one at home, suffering through it. For some women the hospitalization is necessary because the vomiting has led to electrolyte imbalance, dehydration and excessive weight loss. When this severe it can put the baby at risk. Generally though, morning sickness is associated with successful pregnancy outcomes. It is of no help to tell you that despite its prevalence, its cause is still a mystery.
Several theories abound.
However, treating the cause is not an option, but treating the symptoms is. To start, it is important to not add anxiety to the factor. At this point, do not worry about the nutritional intake of your wife, with the exception of a supplement prescribed by her doctor. What will stay down, is what should be eaten. Your wife should find those foods, however few they may be, that she can tolerate, and focus on those.