Researchers at the University of North Carolina at Chapel Hill deliberately exposed volunteers to Norwalk virus, the classic strain of norovirus, to see who would get infected and who would not.
About 29 percent of the volunteers carried two copies of an inactive version of the FUT2 gene, which meant their cells lacked a particular sugar structure the virus normally latches onto. None of those volunteers developed an infection, no matter how large a dose of virus they swallowed.
This genetic trait is called the non-secretor status, and it is permanent and present from birth, not something a person can develop through exposure. It helps explain why some people escape certain norovirus strains that take down everyone around them, though it does not explain every case, since some secretor-positive volunteers also resisted infection, suggesting other layers of immunity are still at work.
Read the original study
Lindesmith et al., Nature Medicine, 2003 · doi.org/10.1038/nm860
Some people are simply built resistant.
Source
- Lindesmith, L., Moe, C., Marionneau, S., Ruvoen, N., Jiang, X., Lindblad, L., Stewart, P., LePendu, J., & Baric, R. (2003). Human susceptibility and resistance to Norwalk virus infection. Nature Medicine, 9(5), 548–53. https://doi.org/10.1038/nm860