Blame "antigenic drift" for the failure of last year's flu vaccine to offer a robust protection against the illness.
The Centers for Disease Control says a series of small genetic changes in a flu virus can make it just different enough from the original variant included in the vaccine, that a vaccinated person's immune system won't recognize it.
And that person can get the flu even after getting the shot.
So, so unfair.
However, health department officials say all was not lost last year.
Jennifer Eisner with the Michigan Department of Health and Human Services says several studies confirmed that vaccinated people who got the flu were significantly less likely to become seriously ill and be hospitalized during the 2014-15 flu season.
And this year, experts believe the vaccine will work a lot better.
"The vaccines available in the 2015-2016 flu season were changed to include the drifted flu virus strain that widely circulated last year, so we do fully expect it to be more effective than last year's vaccine," says Eisner.
There are four confirmed cases of influenza in Michigan now, which means flu season has officially begun.
Eisner says people who want to protect themselves from flu should hurry to get their shot.
"It takes about two weeks for the vaccine to become fully effective in your system, so really, the sooner the better," she says.
Eisner says every year, health officials have to fight the misconception that influenza is not a serious illness.
In fact, hundreds, and sometimes thousands of people die from complications of the flu in the U.S. each year.
Elderly people and those with immune disorders are particularly susceptible, as well as infants under six months, who can't get the vaccine.
People with asthma are also more vulnerable.
During the 2014-15 flu season, three Michigan children died after contracting influenza.
The number of adults who died from flu is not known, because doctors and hospitals are not required to report the incidents to the state.
It's not possible to get the flu from the flu vaccine, but it can still cause some unpleasant symptoms like low-grade fever and aches. Those symptoms usually subside after a day.