Certain blood types have partial or complete immunity to certain strains of norovirus (I forget which ones).

My theory is that we emets take better care to avoid getting sick in the first place, but we can't avoid exposure completely so there's gotta be more to it. My dad (an emet) has only gotten stomach virus TWICE in my life; only once did he vomit. He and I were always the last ones in our house to get sick, IF we did at all. His symptoms were always less severe than the rest of my family's too. Grandpa (another emet) hasn't been v*ed since the 1970s and rarely gets sick at all.

The immune system is amazingly complex. I'm sure there are factors scientists don't fully understand. (I'm always baffled by the spouses of HIV sufferers who never get the disease despite repeated exposures).