MANKATO — Formulating an influenza vaccine is a bit of a guessing game. Scientists do the best they can to determine which strains may come up in the flu virus that circulates through the human population.

But here’s something that doesn’t take any guesswork: It’s a good idea to get immunized because the alternative can be hell.

“If you get influenza, you’re going to be sick. Sick, sick, sick. For a week. You’ll feel like you’ve got meningitis,” said Dr. Richard Peller of the Mankato Clinic. “If you ever get the real flu, you won’t ever forget it.”

Flu shots are the No. 1 recommendation from doctors on how best to avoid contracting the virus. But even the guys who create the vaccine aren’t ever exactly sure what they’ll be up against.

That vaccine is based on what flu strains have circulated in prior years. They learn this by conducting tests on people with confirmed flu cases. Based on those results, they not only get a handle on the strains doing the most damage, but which new strains are coming up. When a strain seems to be picking up stream, they add it to the vaccine.

Sometimes, however, a strain comes along that the scientists never saw coming. This was the case several years ago when the H1N1 strain hit, otherwise known as “swine flu.”

That year, because of the severity of H1N1, the Centers for Disease Control and Prevention decided to issue a second vaccine, and that’s why many people got two flu shots that year.

This year there is new concern about swine flu. Two children contracted a new strain known as H2N3, a strain usually contracted through contact with swine. In addition to the Minnesota cases, there have been more than 200 other cases of H3N2 flu. (H1N1, the swine flu strain from a few years ago, is already included in this year’s vaccination.)

“The shot changes year to year because it is a matter of using the most recent strains,” said Dr. Scott Helmers of Mayo Clinic Health System in Mankato’s Eastridge office. This year there are three strains in the vaccine, but Helmers said next year the vaccine could contain four.

Vaccinations work in several different ways. Injections put inactive viruses into the body that essentially trick the body into thinking a virus is in the house and it needs to fight to get it out. There are also nasal sprays available that apply a modified live virus to the nasal passages. Like the injection, the body gets wind of its presence and sets about the task of eliminating it.

Helmers said it’s a very weakened virus that elicits a very minor infection. This kind of vaccination is good for children, but not children whose immune systems maybe compromised in any way.

The best way to deal with the flu strains is by doing whatever is necessary to avoid contracting them.

n Wash your hands — This is still the No. 1 way to avoid the spread of virus that is often spread from person to person via the hands.

n Avoid crowds — Walking through crowded areas puts you in close proximity with a lot of people. If any of them are carrying the virus, it could spread easily in a crowd.

n Cover that cough — This is a no brainer anyway, but it’s especially important during flu season. Don’t cough on others, cough into your elbow, and avoid chronic coughers.

n Avoid people who have the flu — Another no brainer, but one many people seem incapable of following.

“I would emphasize the hand washing,” Helmers said. “So often we acquire infections hand to mouth.”

In addition to passive ways, people who have contracted the virus can take medication if they’ve got confirmed cases of influenza. The most popular is Tamiflu, which can mitigate the symptoms of influenza but not cure it.

The best advice, though, is to get that shot.

“It’s never too late to get the flu shot,” Peller said.

Influenza kills 36,000 Americans a year.

“Every time I read that number I think, ‘Can it really be that many?’”

Pharmacies are already advertising flu shots. Both the Mankato Clinic and Mayo Clinic Health System in Mankato will have vaccinations available soon.