Everything you need to know about the deadly flu epidemic sweeping the US

Some 37 children have died across the country, and experts think the real number could be double the official reports.

In hard-hit California at least 97 people under the age of 65 have died from this season's flu so far. As of early January, most of those deaths were un-vaccinated adults, the California Department of Health told Business Insider.

But this year has not yet reached the pandemic levels of 2009.

Flu viruses can shift rapidly, making them harder to vaccinate for year to year, and tough to predict. (That's part of the reason why drug-makers have such a tough time coming up with a "universal" vaccine for the flu that would protect you for life.)

The standard for what counts as an "epidemic" changes a little from season to season and even week to week, but the CDC's latest numbers say we've reached epidemic levels. That means the flu is quickly spreading, and so there are more fatal flu cases than what should normally be expected.

The flu reaches epidemic levels (like we're experiencing now) at some point every season. But pandemic flu levels are even more dangerous.

Long before the 2009 swine flu pandemic, there was the deadly 1918 Spanish flu pandemic, which spread around the world, infecting about one in every five people, and killing 50 million. That's more casualties than WW1.

As many as 56,000 people die from the flu every year in the US, the CDC reports. The vast majority of them are elderly and vulnerable patients with compromised immune systems.

People from 50 to 64 years old have some of the highest hospitalization rates right now. The New York Times reports they're sicker than kids, which is unusual.

Coming down with the flu can also make you six times more likely to have a heart attack, according to a new study, because it puts more stress on your system, and can increase inflammation. Plus, "when you get an infection your heart is beating faster," Cardiologist Erica Jones told Reuters.

For seniors, the most vulnerable age group, there's the more potent Fluzone vaccine, which is a higher-dose drug, as well as Fluad and Flublok, which all pack an extra punch.

Scientists say influenza can incubate in the body for 1-4 days before you show any signs of getting sick. That means you can be contagious the day before you start feeling terrible and not know it, as the Centers for Disease Control and Prevention points out.

And some people can spread their virus around without showing any symptoms at all.

Person to person transmission, from talking, coughing, sneezing, or even just breathing on someone else, is more likely to cause an infection than simply touching a surface with some flu virus on it and then reaching for your nose or your mouth.

Doctor Patricia Salber says the flu virus can persist on hard surfaces for up to 24 hours, but it only survives about 15 minutes on tissues.

It's important to wash your hands, and avoid touching your face too much this season to fight the spread of the germs.

New research published in the Proceedings of the National Academy of Sciences this month showed the flu can spread by simply breathing.

Jovan Pantelic, one of the researchers behind the study, said that some people "can exhale or cough high quantities of live virus particles — up to 1,000 in 30 minutes."

"When we consider that an infectious dose is just one virus particle, this means that potentially 1,000 people can be infected in short order," he said in a statement.

A team of researchers from the National Institutes of Health, Harvard and Oregon State tracked decades of flu epidemics and found that the upticks line up almost universally with drops in air humidity.

But simply boosting the humidity in your home isn't a fix-all for illness.

While the flu bug may thrive better in cold, dry air, "there are other pathogens, such as pathogenic mould, that thrive at higher humidity," lead study author Jerrey Shaman told the BBC.

It's true that the flu vaccine is not as effective this year as it has been in the past, but that's largely because the H3N2 strain is everywhere and not well-served by this year's vaccine. It's still a good idea to get vaccinated if you haven't, because the shot is effective at preventing other strains of the virus. Flu season can last into May, so if you haven't gotten your dose yet, it's not too late.

A 2017 study showed that getting a flu shot can significantly reduce a child’s likelihood of dying from the virus. Shots make it less likely you’ll contract the flu, and can also make the illness milder if you do catch it.

Pregnant women are also espeically encouraged to get flu shots, because they're more likely to get hit hard by the flu. Plus, there's some evidence that getting a shot can protect a baby in his or her first months of life, when the tot is too little to get a shot.

Not everyone who gets sick needs to see a doctor. Many people coming down with the flu can stay home and wait it out, with the help of liquids, over-the-counter drugs and a heavy dose of sleep.

But when prescribed quickly, drugs like Tamiflu can help the illness end faster. There have been some isolated shortages reported at pharmacies in urban centers like San Francisco, New York and Atlanta, so it's best to call ahead.

Experts say you can head back to work or school when you've had a full 24 hours fever-free.

Typically, people suffer from the flu for for about a week (5-7 days), but some people can stay sick with the illness for up to two weeks, and often kids can be contagious for more than seven days straight with symtoms like fever, chills, congestion, fatigue, cough and general miserableness.

The canine flu can't spread to humans, but the virus can leap from dog to dog pretty easily.

Some pet owners vaccinate their pooches with a flu shot, but many vets say it's better to monitor your dog for signs of sickness, as the illness isn't usually deadly and is not widespread across the US.