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Explainer: What Is Seasonal Flu?

Health officials recommend getting vaccinated against seasonal influenza, but do you know why? For one thing, the flu is always changing.

Sign that says Flu Season Ahead

Unlike the common cold, influenza can be serious and even deadly. Only vaccines can tackle the respiratory illness, which kills 290,000 or more people globally every year, according to estimates from the World Health Organization.

Compared to a cold, the flu packs a punch with more intense symptoms. Rather than a mild gradual illness, the flu comes on suddenly and can cause complications such as pneumonia and bacterial infections.

Because flu viruses are constantly changing, the flu is a moving target, said Dr. Sarah Cobey, an epidemiologist at the University of Chicago in an article for BBC.

“What's hard about influenza is that it's always evolving into something new that we've never ever seen before,” she said.

The CDC explains that through a process called antigenic drift, influenza viruses mutate (change) the more they’re spread around. The flu spreads through air droplets that circulate when people cough, sneeze or talk. The mutations create new strains and get passed around more in winter - also known as flu season.

Why winter? Cold air and low temperatures helps influenza thrive and spread more easily, explained Peter Palese, flu researcher and chairman of the microbiology department at Mount Sinai School of Medicine in New York, in an article for the New York Times.

Good hygiene and handwashing can help, but humankind’s best strategy against the flu is an annual flu vaccine.

“Immunization is so important to combating influenza, as it is the only proven way to raise your immune defense against the virus,” said Beverly Taylor, Head of Influenza Scientific Affairs at global influenza vaccines provider, Seqirus. “This is why there is a year-round global effort to track and identify the circulating influenza strains, so we are able to develop the vaccines that protect the public against them.”