The deadly flu epidemic sweeping the US is still spreading — here's everything you need to know

AP Photo/Susan Walsh

Flu experts agree that the 2017-18 flu season may not be a full-blown pandemic, but it sure is nasty.

The flu is widespread, across 49 US states right now. Scientists at the Centers for Disease Control and Prevention say that's the first time that's happened in their 13 years of tracking the flu.

The epidemic has been especially tough among baby boomers. CDC officials say that could be in part because they weren't exposed to this year's most aggressive H3N2 strain as children.

It's nearly impossible to predict how this flu season will end, as flu viruses don't follow predictable models from year to year.

Still, there are a few simple things to do to stay healthy and safe. Here's what you need to know about the flu.

It's been a season of horror stories about the deadly, vaccine-resistant flu. On Friday, the Centers for Disease Control and Prevention called this the worst season since the 2009 swine flu.

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Thirty-seven children across the country have died from the flu this year, and experts think the real number could be double the official reports.

In California, at least 97 people under the age of 65 have died. As of early January, most of those deaths were unvaccinated adults, the state's Department of Health told Business Insider.

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

It's been deadly, but this season is not a pandemic.

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

What counts as an "epidemic" changes a little from season to season — and even week to week — but the CDC's latest numbers indicate we've reached epidemic levels, meaning the flu is spreading quickly and leading to more fatal cases than usually expected.

Pandemic flu happens "when a new flu virus emerges that can infect people and spread globally" quickly and efficiently, the CDC says.

National Museum of Health and Medicine

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, which spread around the world, infecting about one in every five people and killing as many as 50 million. That's more casualties than in World War I.

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