Flu is nearly everywhere in America right now. If you’re not sick, there’s a good chance some of your friends, family members, or colleagues are.

Around the country, this flu season brought reports of doctors’ offices and clinics bursting at the seams. In Chicago and the Florida Panhandle, school districts have had to close because so many people were out sick. Alabama even declared a public health emergency over the flu.

Most tragically, seniors age 65 and older are being hospitalized more than any other age group, according to the latest data from the Centers for Disease Control and Prevention. And as of the week ending on February 10, 84 children or infants had died because of flu. Half of those children were reportedly healthy before getting sick, Dr. Anne Schuchat, acting director of the CDC.

“This season, [flu] hit everywhere in the US at once,” Schuchat said in a press briefing Thursday. “We can’t predict how long this season will last —we could be looking at several more weeks of intense activity.”

Flu hospitalizations hit a record high last week since the CDC began tracking them using their current methods nearly a decade ago, and the levels of people reporting flu-like illness across the country are now around as high as the CDC observed at peak of the 2009 H1N1 flu pandemic. “It doesn’t mean it’s a pandemic but just that flu-like illness levels as high as 2009,” she said.

Making matters worse, there’s also an ongoing shortage of saline IV bags to treat the throngs of people who are going to the hospital. That’s because this year’s flu onslaught followed Hurricane Maria in Puerto Rico, where nearly half of the IV bags used in America’s hospitals are made. The hurricane disrupted production, and that coincided with an increased demand for saline because of flu — so hospitals and the Food and Drug Administration are scrambling to address the shortage.

#FDA update on efforts to mitigate ongoing saline shortage during severe flu season. Also, monitoring other critical medical products to prevent shortages, including antivirals, other supportive care drugs & devices: https://t.co/AE9mZYzfck pic.twitter.com/7fOoE53e5d — Scott Gottlieb, M.D. (@SGottliebFDA) February 2, 2018

Flu season started early this year, and for two straight months, the number of cases has mounted steadily. Public health officials anticipate that flu viruses will continue to circulate for a few more months — though the latest data suggest we may be hitting peak flu season soon.

Here’s what you need to know.

Three charts that show this is a really, really bad flu season

This really is a wretched flu season. Just look at these three data visualizations.

This GIF shows flu activity — from sporadic to widespread — for February 2018, compared to the past two years:

According to the latest CDC report, as of February 10, Puerto Rico and 48 states were still experiencing widespread flu activity.

The number of people turning up at hospitals for what they think is flu is a good proxy for how bad things are. You can see in red in the chart below the percentage of outpatient hospital visits for flu or flu-like illness last year and in early 2018:

The bad news is that this flu season is an absolute outlier among recent years for people turning up in hospital for flu. The good news is that the number of flu-related hospital visits appears to be plateauing.

Across the country, 7.5 percent of patient visits, reported through the CDC’s US Outpatient Influenza-like Illness Surveillance Network, were due to influenza-like illness. This is down from 7.7 percent last week — which exceeded the previous record high (7.6 percent) for a non-pandemic year, during the 2003-’04 flu season.

Adults 65 or older are experiencing the highest rate of lab-confirmed flu hospitalizations, followed by adults ages 50 to 64, and infants and children ages 0 to 4:

The rate of lab laboratory-confirmed flu hospitalizations hit 67.9 per 100,000 population as of February 10. That’s higher than the previous record, in 2014-15, of 64.2 per 100,000.

During a typical flu season, the number of pediatric deaths can range from 37 to 171. There have been 84 so far this season, but flu season could run as late as May.

Why this flu season is so bad

Flu is a viral respiratory disease, and it leads to nasty fevers, headaches, coughs, muscle pains, and runny noses that make many people miserable in the fall and winter.

There are four species of influenza viruses — A, B, C, and D — and seasonal flu is caused by influenza A and B. Every year, different strains of these viruses circulate. The reason this year’s flu season is more severe than usual is because it involves the dreaded H3N2, a strain of the influenza A virus that causes more health complications and is more difficult to prevent.

H3N2 hits people harder than other seasonal flu strains and can be especially deadly among vulnerable groups like the elderly and children. Researchers still aren’t sure why, but they’ve found that a flu season involving the H3 virus is generally nastier — with more hospitalizations and flu-related deaths — than seasons involving mostly H1N1 or influenza B viruses.

This year, the majority of flu cases involve H3N2 strain.

It’s not yet clear what the final toll of this year’s flu season will look like. But for context, in recent years, mild flu seasons tend to kill about 12,000 Americans, and severe flu seasons kill about 56,000.

What happened in Australia, where the flu season peaks in August, might be a hint of what we’re in for here. H3N2 struck there as well — and contributed to more than two and a half times the number of flu cases compared to the previous year. There were also 745 deaths, compared to the five-year average of 176 deaths, according to the Sydney Morning Herald.

H3N2 is especially difficult to prevent with the flu shot

Another reason having the H3N2 strain in circulation is so nasty is because it’s hard to prevent with the flu vaccine.

To understand why, you need to understand how the flu vaccine works — and why it is by no means perfect.

The vaccine is designed to protect people against three or four strains of the A and B viruses that researchers believe will be most common in a given year. So every year, public health agencies essentially make educated guesses on what strains and mutations will make the rounds. As you can imagine, this is no easy task — and in reviews of the research on flu vaccine effectiveness, researchers have found that years when H3N2 is circulating tend to be years when the vaccine is less effective.

Edward Belongia, a senior epidemiologist at the Marshfield Clinic Research Institute in Wisconsin who has studied flu vaccine effectiveness, found that the combined vaccine effectiveness during H3N2 seasons was 33 percent, meaning getting vaccinated reduced a person’s risk of having to go to the doctor by about a third. The flu shot’s effectiveness rose to 54 percent during influenza type B seasons, and 67 percent during H1N1 seasons.

The first data on how well the flu vaccine is working this season in America has just been published — and it suggests the vaccine may be faring even worse than in previous years.

The study, from the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report, tracked flu cases among 1,700 children and adults across the US. They found the flu shot was 36 percent effective overall, meaning it reduced a person’s risk of getting sick with flu and going to a doctor’s office by about a third.

But the vaccine’s effectiveness against H3N2, the most commonly circulating strain this flu season, was much lower: 25 percent. And this finding squares with an earlier report from Canada, which found the flu vaccine was only 17 percent effective against H3N2.

As I’ve explained, there are a couple of reasons why H3N2 seems to be particularly resistant to the flu shot. H3N2 mutates as it moves through the population at a faster rate than other flu viruses — making it even harder to design a vaccine that can fend off the circulating virus. It’s also more difficult to grow H3N2 in eggs, where viruses for flu vaccines are produced. “In the process of adapting virus to grow in eggs, that seems to introduce further changes to the virus, which may impair the effectiveness of the vaccine,” Belongia explained.

It’s not too late to get a flu shot

Still, if you haven’t done it yet, it’s a good idea to get the flu shot anyway. Flu season will last several more months and could even stretch into May.

Even if this year’s vaccine isn’t a good match for H3N2, it offers better protection against other flu viruses going around. (Again, H1N1 and influenza B are more easily prevented with the vaccine — so getting vaccinated now can reduce your risk of these other infections even if it won’t necessarily prevent H3N2. And influenza B infections are on the rise across the country.)

As I’ve reported before, flu vaccines also carry very little risk — and the potential benefit of avoiding serious illness. Belongia argued that even some protection is better than no protection. “That still prevents a lot of hospitalizations and deaths,” he said.

Most people can just wait out the flu at home, getting lots of rest and taking care not to infect others. But certain people in high-risk groups — young children, pregnant women, people over age 65, people with other chronic diseases — may want to seek medical attention and antivirals like Tamiflu.

In the meantime, there are other, even simpler things you can do to mitigate your risk of getting the flu and, if you do get sick, spreading the virus to others. They include good old-fashioned hand-washing, covering your mouth when you cough, and staying home if you’re sick.

Correction: An earlier version of this post misinterpreted a CDC chart about the burden of flu hospitalizations among certain age groups.