You got a flu shot. You’ve kept your hands away from your mouth, eyes and nose, and you’ve kept yourself away from sick co-workers.

So how come you still came down with the flu?

The plain truth: It isn’t easy to protect against this tenacious, changeable virus.

“The more we learn about influenza, in many ways the less we know,” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota’s School of Public Health.

Uncertainty about how viruses are passed and which strains will be active from year to year makes prevention a challenge. To complicate things, some habits that contribute to better overall health may do little to protect against the flu, which is caused by a virus that infects the nose, throat and lungs. More severe than a cold, it can lead to hospitalization or, in rare cases, death.

A delicate virus

Most experts believe that flu viruses spread mainly by moist droplets made when people sick with the flu cough, sneeze or talk. These droplets can land in the mouths or noses of people nearby.

Less often, a person also could catch the flu by touching a surface or an object that has the virus on it and then touching their mouth, eyes or nose. But unlike cold viruses, flu viruses are fragile and don’t survive for very long outside the body, Osterholm said. That’s why some common practices that can help prevent against other viruses may not be effective against the flu.

“There have been several studies done looking at hand washing and hand sanitizing with influenza,” he said. “The data are quite unclear, with more data suggesting that hand washing may play a very, very limited role with influenza.”

While Osterholm still recommends frequent hand washing, he said it won’t keep the flu away.

“We can prevent a lot of other infectious agents including food-borne pathogens and other respiratory agents with hand washing,” he said. “So I’m very high on hand washing. I just think the attributable prevention factor for flu has been oversold.”

According to Osterholm, other healthy habits also have not proven to effectively guard against the flu. Other health officials disagree.

Eating a healthy diet, sleeping well and getting enough exercise can help to prevent infectious diseases — including the flu — by supporting a healthy immune system, said Dr. Greg Poland, director of the Mayo Clinic’s vaccine research group.

Poland and Osterholm do agree that the flu vaccine remains the best weapon available, even though it has its limitations.

Better, but not perfect

Several recent studies suggest that flu shots may not save the lives of as many adults age 65 and older as ­previously thought.

“The whole idea that the flu vaccine protects 90 percent of the time is just not true,” Osterholm said. “It varies by age, by underlying risk group, and it varies by year.”

While there are studies that clearly show that the live vaccine works well in children, there is no evidence that it’s as effective in adults, he said.

Last year’s flu shot, for example, was determined by federal officials to be only 9 percent effective among the elderly, who were hardest hit by the flu outbreak.

But there have been some improvements in the vaccines.

This year, certain vaccines will guard against four strains of flu instead of the usual three. For healthy people ages 2 to 49, there also are nasal sprays that protect against all four flu strains. (The nasal sprays, which contain live viruses, are not suitable for those younger than 2 and older than 49 because their immune systems tend to be weaker.) There also are specialized shots, including one that penetrates just below the skin for those who don’t like needles; a higher-dose shot for people age 65 and older; and one that’s grown without eggs for those with egg allergies.

In the Upper Midwest, flu season typically runs from October through May. Flu shot promotions are in full force now, but flu activity in Minnesota has so far been at a very low level, said Kris Ehresmann, director for infectious disease at the Minnesota Department of Health.

That could change. Flu activity typically peaks in mid-January.

Ehresmann recommends getting vaccinated before then, because it takes two weeks for the vaccine to be effective. She tries to explain that getting a flu shot, no matter how imperfect, isn’t just about protecting individuals from contracting the flu. It’s about stopping the virus from spreading and possibly becoming a pandemic.

She’s also hopeful that the weapons in the fight against the flu will only get better.“In the last three or four years, we’ve learned a lot more about influenza,”she said. “There’s a greater attention to influenza, the vaccine and its effectiveness. We’re learning all the time and the important message is that new data keeps coming out.”