The flu is widespread and active in the US. Here’s what you need to know.

The US is currently experiencing an unusually nasty flu season, with 49 states reporting widespread activity.

Seasonal influenza hits the US every winter, but you've probably heard that this year's season is particularly dangerous. There have been reports of the flu causing overcrowded emergency rooms and medicine shortages at pharmacies. But just how bad is it?

According to the latest weekly influenza surveillance report from Centers for Disease Control and Prevention (CDC), the flu is already widespread in 49 states and it has killed over 100 people so far, including 20 children. "Right now, flu activity is elevated and intense, and the CDC has classified the severity of this year's flu season as moderate," Lynnette Brammer, lead of the CDC's domestic influenza surveillance team, told BuzzFeed News.

"Flu is everywhere in the US right now," Dan Jernigan, director of the Influenza Division of the National Center for Immunization and Respiratory Diseases, told reporters in a briefing on Friday. Currently, states in the West and South (such as California and Georgia) are experiencing the highest levels of flu activity — but other Northern states may peak soon, too.

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The 2017–2018 flu season started early, and spiked around the holidays.

"This year's flu season had an early start, with a lot of cases accumulating in the past month and more hospitalizations and deaths in certain regions of the country compared to last year — this is a tough year," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID), tells BuzzFeed News.

The spike in flu cases started around the holidays, when domestic travel — and therefore, the spread of germs — was at an all-time high and the nation experienced severe cold weather.

The CDC estimates that since 2010, there have been between 140,000 and 710,000 flu-related hospitalizations and 12,000 to 56,000 flu-related deaths. Each year is different, but the season typically peaks around February, so this year's season seems to be peaking relatively early.

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The prevailing strain, H3N2, is not new — but it is known for being particularly vicious and severe.

The US has actually seen H3N2 many times before — even as recently as last year, Brammer says. There are four types of influenza: A, B, C, and D, but it's influenza A and B that typically cause seasonal flu epidemics among humans. "Each flu season we usually see influenza A subtype H3N2 or H1N1, and one or more influenza B viruses," Fauci says. About 82% of cases this season are of the H3N2 strain, which is not a good thing.

"When H3N2 is the predominant strain, that usually means it will be a severe flu season — H3N2 is inherently bad," Fauci says. That's because H3N2 has a greater propensity to cause flu-related complications, which can lead to hospitalizations or deaths, especially among the elderly and very young.

In addition to H3N2, surveillance data show that H1N1 and influenza B are also circulating this year, Brammer says, which is normal.

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The pattern of this year's flu season is actually looking similar to the 2014–2015 season, but things could change in the weeks to come.

The 2014–2015 flu season also had an early peak with high levels of flu activity in January, Fauci says, and the predominant strain was H3N2. "If you look at the CDC's FluView Index, the infection rate for this year's season is tracking very similar to the 2014–2015 season," Brammer says. There will likely be geographical differences in this season, but the overall numbers are not abnormal when compared to past years.

So while this flu season is awful, it is not outside what we’ve seen in other years when H3N2 is the predominant strain. "The number of flu-related hospitalizations is actually lower than what we saw this time of year in 2015, so we do expect there to be fewer deaths this time around," Fauci says. The number of deaths in children this season is also much lower than last year, Fauci says, but this number could go up.

All that being said, the 2017–2018 season is nowhere near over and the flu is unpredictable — so the season could end earlier or later than expected. "Even if we are at the peak now, we still have a ways to go with flu season, so things could change in the weeks to come," Brammer says. (Flu season typically ends in March in the US.) It's up to states to report flu-related deaths and send the data to the CDC, which can take time, so experts won't know exactly how bad this season is until it's over.

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However, it's unlikely that this year's season in the US will mimic the "killer Aussie flu."

The "Aussie flu" is another name for the strain of H3N2 that hit Australia during the country's winter months in 2017. According to the Australia Government Department of Health, there were 233,453 confirmed cases of the flu and 745 flu-related deaths, which was a dramatic increase from the previous year.

Australia's deadly flu season sparked concern that the US flu season would be equally bad if not worse. However, Fauci says that this is unlikely. "Australia does not have as much broad protection against H3N2 as the US for two reasons: First, in Australia, they only give the flu vaccine to high-risk groups such as the elderly or health care workers. Second, Australia hadn't seen much of H3N2 before, so a lot of people had never been exposed," Fauci says.

In the US, the CDC generally recommends that everyone over the age of 6 months gets the seasonal flu vaccine. The H3N2 subtype that hit Australia has been circulating in the US for years, so more of the population has developed at least some immunity. "Although the flu is hitting this year with a vengeance, we are still likely to do better than Australia," Fauci says.

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This year's seasonal flu vaccine protects against several strains of the flu, but it's unclear how effective it is against H3N2.

The seasonal flu vaccine usually protects against three or four strains of the flu, which are chosen based on predictions for what will circulate in the US that year. According to the CDC, this year's flu shot included an influenza A H1N1, an influenza A H3N2, and an influenza B/Victoria lineage virus. So if H3N2 was included in the flu shot, why is it still making people sick?

H3N2 is, historically, a vicious strain of flu. So if it's the predominant strain, that season can still be severe even if people have some background exposure or immunity, Fauci says. H3N2 also mutates quickly, so the "reference strain" included in the flu shot may differ slightly from the H3N2 strain that shows up during flu season, lowering the vaccine's effectiveness.

“Even at its best, the seasonal flu vaccine is only 60% effective, and we’ve had years where it’s as low as 10% but we anticipate it won't be that low this year,” Fauci says. Experts won't know how effective the 2017–2018 flu vaccine is until the flu season is over, but the experts are predicting that it will be around 30% effective. This is much higher than Australia's flu vaccine, which was only 10% effective against H3N2.

That being said, the flu shot also protects against H1N1 and influenza B, which are circulating this year and tend to have a higher vaccine effectiveness than H3N2, Brammer says.

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The people at highest risk for flu-related hospitalizations and deaths are still the elderly, the very young, and those with chronic illnesses.

The flu can kill you in a few different ways. It either leads to a secondary bacterial infection (such as pneumonia), overwhelms the body's immune system, or causes organ failure. "People usually die from respiratory failure, which leads to organ failure because when the lungs aren't working, there isn't enough oxygen in the blood going to the rest of the body," Fauci says.

"When we look at the rate of hospitalizations and deaths, the pattern is still holding true that it's killing mostly those who are 'high risk,' such as the elderly, immunosuppressed, and people with chronic health conditions," Fauci says.

It is true that this year's flu has killed and continues to kill young, healthy adults, but the experts say these are outliers. "These cases are very shocking and tragic, but they are still the overwhelming minority of cases," Fauci says.

"We aren't seeing anything in our data would make us think that, the pattern of mortality in terms of age groups is any different from other H3N2 years," Brammer says. The cases of young people dying do speak to the fact that the flu is a deadly disease and it should be taken seriously.

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The best way to protect yourself is to get a flu shot if you haven't already. It's not too late.

The flu shot is not perfect, but it's your best defense. The CDC is still recommending that anyone over the age of 6 months gets a flu shot, because the flu will continue to circulate for several more weeks. "We could have a second wave of Influenza B after this H3N2 wave passes though, so there's still potential to benefit from the vaccine," Brammer says.

Sure, the flu vaccine might not be super effective against H3N2, but it's better to have some protection against all of the circulating strains than no protection at all. "Even when the shot fails to prevent disease and you get the flu, the shot can mitigate illness so you have a less severe, shorter course," Fauci says. In short, the flu shot can keep you out of the hospital. And no, the flu shot cannot give you the flu.

It's not too late, but you should get it sooner rather than later because it often takes the body two weeks to develop antibodies that protect against the virus. Talk to your doctor about whether the flu shot is safe for you and which type you'll need. Use the CDC’s Flu Vaccine Finder to find out which pharmacies and clinics are offering the shot in your area.

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If you think you have the flu, go to the doctor — especially if you are considered high risk.

If you have flu symptoms — a fever and/or chills, body aches, coughing, runny nose, sore throat — or suspect you have the flu, go to the doctor as soon as possible. Antivirals like Tamiflu can lessen the severity and shorten the course of the flu, but they're more effective the sooner you take them.

This is especially important for people who are high risk, such as the elderly, children, pregnant women, or those with a chronic disease, Fauci says. These groups are particularly vulnerable and can benefit the most from antivirals. "Don't wait until you feel really bad," Fauci says. The flu moves fast and can kill within days after the onset of symptoms.

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