Why do some people seem under the weather all the time, while others can share a bowl of chicken soup with a flu patient without getting the least bit sick? It all comes down to the immune system. The strength of this hardware inside us to fight off disease varies considerably from person to person, and is affected by a number of factors, like age and nutrition. So it seems reasonable to think that genetics play a role, too. But how much of one? Stanford researchers tackled that question in a study published Jan. 15 in the journal Cell. They took various immune system measurements in 105 sets of healthy twins, to separate genetic from non-genetic factors. They found that that while heritable factors do influence the immune system somewhat, non-heritable factors are more important. This was not the finding the researchers expected. Mark Davis, senior author of the study, told Scientific American that his team was “surprised by the degree of environmental influence on so many components.” More via Quartz.