It depends on the symptoms. Apparently this one is a nasty pneumonia that fills the lungs with fluid. A 32-year old UK National backpacking in Thailand (presumably fit person) was “at death’s door” according to his father. He has since improved.
I think a significant majority of people experiencing those symptoms will seek care at some point. But some might not actually get to that point, so yeah, not everyone infected will become a confirmed case.