The holidays are all about fun, family, food, and togetherness. Unfortunately, this togetherness seems to have encouraged the rise of influenza throughout the Southern United States.

According to influenza data published weekly by the Centers for Disease Control and Prevention (CDC), there have already been 1.7 million cases of flu illnesses reported in the U.S. this season, which go along with approximately 16,000 hospitalization and 900 flu-related deaths. We have to go all the way back to 2003-2004 to find the last time that an outbreak of the flu ramped up this early in the season. 

Anytime influenza cases are ramping up, worries and misconceptions often arise. With that being said, here are 10 things you need to know about the 2019-2020 flu season.

A sick young woman under the blankets with a thermometer in her mouth, a cup in her left hand, and over-the-counter medicine packets flanking her.

Image source: Getty Images.

1. The flu is most prevalent between December and February

Although we're getting an early start to the 2019-2020 flu season, this is traditionally an ailment that peaks during the winter months. Between the 1982-1983 and 2017-2018 flu seasons, February marked the peak month of flu illnesses 15 times, followed by December (7), January (6), and March (6). November and October each saw the year's peak number of flu illnesses just once. While it's unlikely that we'll see any early peak to the current flu season, it's not unheard of. 

2. It's unclear why the flu is so virulent in the winter months

Sometimes the best answer that can be given is the admission that no one knows. The honest truth is that researchers aren't entirely certain why influenza always ramps up its attack during the winter months. One possible thesis is that, since children have weaker immune systems than adults and haven't been exposed to as many influenza viruses throughout their shorter lifetimes, schools act as a breeding ground for transmission. However, this hypothesis still doesn't explain how certain geographic regions of the U.S. see outbreaks, while others do not. 

3. Up to 45 million people contract the flu each year

Just because researchers can't explain why influenza cases ramp up each year, it doesn't make instances of the flu virus any less unpleasant. According to the CDC, between the 2010-2011 season and 2017-2018 season, 9.3 million to 45 million people contracted the flu each year, with between 140,000 people and 810,000 people being hospitalized annually due to their illnesses. On average, anywhere from 25 million to 30 million people will come down with the flu each year. 

A physician holding his glasses while in deep thought.

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4. It's the eighth leading cause of death in the U.S.

For most Americans, the flu is a nuisance that'll keep up us in bed for a few days, and perhaps take a full two weeks before we're feeling like our old selves again. But for the elderly and young children whose immune systems aren't as strong, the flu can be a death sentence if the proper precautions aren't taken. Between 2010-2011 and 2017-2018, anywhere from 12,000 people to 61,000 people died annually from a flu-related illness. Per the CDC, the flu/pneumonia is the eighth leading cause of death in the United States. 

5. A flu shot can't give you the flu

Arguably the biggest misconception about flu season is that getting a flu shot is going to give you the flu, which is completely false. With the exception of one recommended flu medication, the vaccines use inactivated strains of the influenza virus. Essentially, these inactive strains serve as a teaching tool for your immune system to help train it to fight influenza more effectively. They absolutely can't give you the flu.

Meanwhile, the other flu medication, AstraZeneca's (NASDAQ:AZN) intranasal spray known as FluMist, utilizes attenuated flu virus strains. This is a scientific way of saying they're live strains that have been substantially weakened. Thus, getting intranasal FluMist could give you the symptoms of a cold, but not the full-blown flu. I'll have a bit more to say about FluMist in a later point.

A physician holding a clipboard while having a discussion with a female patient.

Image source: Getty Images.

6. A flu shot also can't prevent you from getting the flu

Another common misconception is that if you do get a flu shot you'll be completely protected from catching the influenza virus. While researchers do an incredible job of forecasting what sort of strains could be active in an upcoming flu season, it's really just a glorified guessing game that sometimes turns out poorly. You see, there simply wouldn't be enough influenza vaccine to go around if pharmaceutical companies waited until they knew what strains were active during the winter season. Making an adequate number of vaccines takes months, which is why a flu vaccine isn't always effective.

So why get one? The primary purpose of the vaccine isn't to prevent a person from getting the flu so much as to reduce the potential of them winding up in the hospital. If the vaccine "teaches" their immune system how to respond to certain strains, it could mean the difference between a milder form of the flu and winding up in the hospital. On average, fewer than 50% of Americans get a flu shot each year. 

7. Quadrivalent vaccines dominate the landscape

Although there are a number of different influenza vaccines, quadrivalents are what tend to dominate the landscape. Quadrivalent vaccines contain two inactivated type A viruses and two inactivated type B viruses, thereby offering the best learning tool for your immune system. Type A influenza is what leads to pandemics, can be the most deadly to seniors and children, and often dominates earlier in the flu season. Meanwhile, type B influenza tends to be milder and usually hits later on in the flu season.

A pharmaceutical lab researcher using a microscope and pipette.

Image source: Getty Images.

8. This is a roughly $6 billion indication

For you investors out there, yes, influenza can be a big-dollar indication. Back in 2017, BCC Research labeled the global influenza market as being worth $5.6 billion annually and foresaw a compound annual growth rate of 3% through 2022. This would make the worldwide flu market worth about $6 billion today. Of course, keep in mind that Big Pharma isn't strictly devoted to influenza, with flu vaccines generally playing a complementary (rather than leading) role in revenue generation. 

9. Sanofi is the influenza vaccine kingpin

However, there is one pharmaceutical company that's done a particularly good job of cornering influenza vaccine market share: Sanofi (NASDAQ:SNY). During the fourth quarter of 2018, Sanofi wound up generating $663.5 million in sales from influenza vaccines (69% of which was in the U.S.), representing 39% of vaccine sales during the quarter, and nearly 7% of the company's total revenue in Q4. Historically, FluZone has done the heavy lifting for Sanofi, but the launch of its new quadrivalent vaccine Flublok has added extra pep to the company's vaccine sales and bottom line. 

10. FluMist is back as a recommended option

Lastly, as I promised to discuss, AstraZeneca's FluMist is one again back as an option for consumers. In 2016-2017 and 2017-2018, AstraZeneca's intranasal option was removed from the CDC's list of recommended influenza options due to a lack of efficacy. But following a reformulation and relaunch from the company, this marks the second consecutive flu season that it's back as one of the many recommended options for consumers -- especially those who are afraid of needles. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.