Sure enough, a few days later Bennett had the runny nose and achiness of mild flu.
The best defense against influenza is a yearly vaccine, but it's far from perfect. In fact, the vaccine is least effective in people age 65 and older — the group most susceptible to flu — probably because the immune system weakens with age.
Understanding how younger adults' bodies fight flu may help scientists determine what the more vulnerable elderly are missing, clues to help develop more protective vaccines for everyone, Memoli explained.
Here's the issue: The vaccine is designed to raise people's levels of a particular flu-fighting antibody. It targets a protein that acts like the virus' coat, called hemagglutinin — the "H'' in H1N1, the strain that caused the 2009 pandemic and that is causing the most illness so far this winter, too.
But it's not clear what antibody level is best to aim for — or whether a certain amount means you're protected against getting sick at all, or that you'd get a mild case instead of a severe one.
"As mind-boggling as it is, we don't know the answer to that," said Dr. Anthony Fauci, chief of NIH's National Institute of Allergy and Infectious Diseases. "We made some assumptions that we knew everything about flu."
Just targeting hemagglutinin probably isn't enough, Memoli added. Already, some people in his study didn't get sick, despite remarkably low antibody levels, meaning something else must be protecting them.
Could it be antibodies against the "N'' in flu's name, the neuraminidase protein? Specific T cells that are activated to fight infection? Genes that switch on and off when a virus invades?
To begin finding out, Memoli first developed a laboratory-grown copy of the H1N1 flu strain and sprayed different amounts into volunteers' noses until he found the right dose to trigger mild flu. He hopes eventually to test the harsher H3N2 strain, too.