Why do some people end up in bed feverish, hacking and sneezing for days from the flu — when others seem to never get sick?

To answer that question, University of Michigan researchers did the first study of its kind: They infected 17 healthy people with the flu virus and discovered that everyone who is exposed to the flu actually is affected by it, but their bodies just have a different way of reacting to it. Half of the study participants got sick; the other half didn’t notice a thing.

“Many people might conclude that if you are exposed to a virus and you don’t get sick, it’s because the virus didn’t stick or it was so weak, it just passed right through your system and your system didn’t notice. That’s not a correct notion,” says Alfred Hero, professor at the University of Michigan College of Engineering and author of the study, which was published Thursday in the journal PLoS Genetics.

He continues, “There is an active immune response which accounts for the resistance of certain people getting sick, and that response is just as active as the response we all know and hate, which is being sick with the sniffles, fever, coughing and sneezing. It’s just that the responses are different.”

Hero, along with scientists from Duke University Medical Center and the Duke Institute for Genome Sciences & Policy, studied participants’ gene expression to watch how the immune system reacted to the flu virus. The analysis reviewed 22,000 genes and 267 blood samples, and used a pattern recognition algorithm and several other methods to discover the genomic signatures associated with the immune response in people who get flu symptoms and those who do not.

They found significant and complex immune responses in the people who got sick and the people who didn’t. Scientists noticed changes in their blood 36 hours before some people actually felt sick. Although they understand that some people’s immune systems resist the virus, they still don’t know how or why that happens.

“There is a behind the scene active immune response even when you don’t get sick,” Hero says. “What we found were differences in their biological metabolism and gene expression. These differences had to do with antioxidants.”

Eventually, if scientists can understand what happens at the level of the genome that makes people more or less susceptible to viral illness, they could potentially develop therapies to prevent the illness.

Lamar Johnson, 44, of Minneapolis, says he’s often wondered if drinking several glasses of juice daily and eating loads of fresh produce keeps him healthy — because in the dead of winter, when everyone around him seems to be suffering from the flu, he never gets sick. In fact, he doesn’t believe he’s ever had the flu in his life.

“I just stay out of the cold, do my best to stay away from sick people and hope I don’t get sick,” he says.

Indeed, Hero says drinking juice, and eating fresh fruits and vegetables to load up on antioxidants may be the answer to avoid getting sick with the flu.

“It’s certainly possible that people who came in had a very high level of antioxidant precursors in their blood, and this may what protected them, but we’re not saying that because we don’t know. You can’t go beyond the data to make these hypotheses.”

Researchers know the flu can be deadly. The Centers for Disease Control and Prevention reports that of the 15 million to 60 million Americans that get the flu each year, and 5,000 to 45,000 die from it. About 200,000 people end up in the hospital with the flu each year.

Hero says his research could lead to an inexpensive test people could take to tell if they are going to have flu symptoms 36 hours in advance. Hero says, “That way people would know if they need to take time off, cancel their ski vacation or isolate themselves from their grandparents who are very susceptible.”