How worried should you really be about all the fearsome-sounding flu strains in the news?

With all the fearsome-sounding flu strains in the news——bird, swine, and all those letter-number combinations——it's easy to make yourself sick just thinking about them. And it's true: Influenza is a highly contagious, rapidly transforming pathogen that keeps doctors and pharmaceutical companies guessing every year.

"Influenza viruses share a similar genetic makeup, and they all infect the respiratory system," says Robert Salata, MD, chief of infectious diseases at University Hospitals Case Medical Center in Cleveland. "But there are several types and subtypes, some of which are more of a threat than others." There are four types of flu viruses, called A, B, C, and D, some of which can be further categorized into subtypes or at least specific strains. Here's all you need to know about what could be circulating this winter.

Seasonal flu

Scary stats: Although regular old influenza may not seem as frightening as some of its more exotic siblings, the flu hospitalizes between 140,000 and 960,000 Americans a year and kills anywhere from 12,000 to 79,000 annually in the U.S. "Seasonal flu" actually refers to several circulating strains of influenza, says Lisa Grohskopf, MD, medical officer for the CDC. Seasonal flu is usually caused by an influenza A or influenza B virus. (Influenza C infections are usually only mild, while D viruses are not known to infect humans.) Thanks to flu trackers, scientists can predict fairly accurately which strains will be most common in the U.S. each year.

What it's like: For most healthy adults, seasonal flu lasts about a week and usually goes away on its own. It tends to be more debilitating than the common cold, with symptoms like head and muscle aches, fever, cough, sore throat and a runny nose. Young children, pregnant women, and adults 65 and up——plus those with a compromised immune or respiratory system——are most vulnerable to complications.

How it spreads: Usually through the air; less commonly via contact with a contaminated surface.

Should you worry? Yes. All Americans are at risk of a bad case of seasonal flu. Defend yourself by following common preventions, like washing your hands or getting the flu vaccine.

H1N1 flu

Scary stats: Formerly known as swine flu, this influenza A virus surfaced in 2009 and reached pandemic proportions, infecting approximately 60 million people in the U.S. (and killing an estimated 12,000). It has returned every year since, but a vaccine has helped decrease infection rates, and it's now considered a form of seasonal flu.

What it's like: Symptoms are similar to those of other seasonal flu strains, although unlike other strains, H1N1 has been shown to hit healthy adults under 65 especially hard.

How it spreads: It's transmitted in the same ways as other seasonal flus.

Should you worry? H1N1 could be prevalent again, but if you get your general flu shot, you'll have some good protection: Both the trivalent and quadrivalent versions of the flu vaccine include H1N1.

Avian flu

Scary stats: In 2014, a Canadian woman who had traveled to China died after contracting H5N1, the same strain of bird flu that has infected more than 800 people (and killed 440) in Asia and the Middle East since 2003.

What it's like: When humans contract H5N1, also a type of influenza A virus, they can develop fever and cough and have difficulty breathing.

How it spreads: Both of the most common forms of avian flu spread mainly from infected birds to humans.

Should you worry? Not unless you're traveling in countries with outbreaks (including China, Egypt, and Indonesia) and hanging around poultry. You can't catch it from eating cooked chicken or handling meat bought at a supermarket.

Variant flu

Scary stats: When a virus present in pigs makes its way to humans, it's considered a variant virus. Since 2011, more than 400 people in the U.S. have been infected by H3N2v, mostly after exposure to pigs at state fairs. A vaccine is being developed.

What it's like: For most people, H3N2v feels like a mild seasonal flu, but complications can occur. The majority of those infected have been children, not adults, probably because H3N2v is related to human flu viruses from the 1990s, so grown-ups tend to have a built-in immunity.

How it spreads: Humans can become infected via exposure to a sick pig if, for example, it sneezes and a droplet lands in your nose or mouth.

Should you worry? Probably not. Like bird flu, H3N2v doesn't appear to circulate easily among humans, and you can't catch it from eating properly cooked pork. If you do spend time around pigs, wear gloves, wash your hands afterward, and avoid any swine that seem to be obviously, well, under the weather.

How to prevent the flu

Get a flu vaccine. Encourage friends and family to get one as well. Go for a quadrivalent vaccine (in either shot or nasal spray form) if it's available, since it offers protection against four flu strains rather than three. But if your provider has only the trivalent, get it.

Wash your hands often. Use an alcohol-based hand sanitizer when you're not near a sink, and avoid touching your eyes, nose and mouth——especially after shaking hands or handling doorknobs, railings, or keyboards.

Stay home if you're sick. You'll avoid infecting friends, co-workers, and fellow commuters. You can pass the virus on to others for five to seven days after first showing symptoms.

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