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"It's just the flu."

I can't tell you how many times I've heard that expression. Yet each time I hear it, I cringe.

"Just the flu" is responsible every year for hundreds of thousands of hospitalizations and thousands of deaths. It is a disease that makes those of us who work in public health very nervous.

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The only real certainty is that there will be a flu season each winter. Other than that, there are many unknowns: When will it start? What strains will be circulating? Who will be at greatest risk? How severe will it be? How well will the vaccine work?

This year's flu season is now in full swing so we have some of the answers to such questions.

We know that the number of flu cases started to spike first in the southeastern United States. Visits for flu-like illness started to rise in December and have continued to rise and spread. According to the Centers for Disease Control and Prevention, 20 states have high levels of flu activity and all regions of the country are now seeing flu cases. The website, Google flu trends, which tends to be more up to date, reports that 33 states are seeing high levels of flu.

More than 90 percent of disease is caused this year by the H1N1 strain of the flu virus. This is the same strain that also caused the so-called swine flu pandemic in 2009. It has been included in all flu vaccines since then.

READ: This Year's Flu Hitting Young and Healthy

H1N1 tends to hit adults between the ages of 18 and 64 harder than other flu strains. But groups traditionally at high risk - the very young, the elderly, those with chronic diseases, pregnant women - haven't been spared.

Although the flu season is in full swing, it often continues into March. It is too soon to know how severe this flu season will be, how long it will last and how well the vaccine will work. We can only tell these sorts of things when the flu season has ended.

The best flu prevention? Get vaccinated. Flu vaccination is recommended for everyone older than 6 months. Getting vaccinated does two things: It reduces your chance of getting the flu and helps protect those around you from getting it by creating something called "herd immunity." This means that even those who don't get vaccinated are protected because the virus has fewer ways of spreading in a population of mostly vaccinated people.

There are more vaccine choices this year than ever before. One shot will cover you for four strains of flu, rather than the usual three. There is a high-strength vaccine specially developed for the elderly, a group for whom standard vaccines have typically been insufficiently effective. There is a nasal spray vaccine for people 50 and younger. There is now even a vaccine made without the use of eggs for anyone with egg allergies.

You can also prevent the spread of the flu by covering your coughs and sneezes with a tissue or with your elbow. Washing your hands frequently with soap and water for a full 20 seconds also helps gets rid of those viruses. If there's no sink around, alcohol-based hand sanitizer is a suitable substitute.

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And if you're sick, stay home. Set an example so maybe others will do the same.

And please, whatever you do, don't say "it's just the flu."

Flu Tweet Chat Today at 1 p.m. ET

Join me today for a tweet chat to find out everything you need to know to protect you and your family from the flu this year. We'll be joined by numerous experts from top hospitals and research centers all over the country including the CDC and the Mayo Clinic.

Joining the chat is easy. Find out how you can participate in 3 easy steps.