On Wednesday, the first reported incident of flurona since the term became popular showed up in the U.S., where an unvaccinated teenager in Los Angeles tested positive for both COVID and the flu, marking flurona’s official arrival in the U.S. However, there were other reported instances of flurona before that. 

Here’s how the latest public health threat is impacting patients around the world.

What is flurona?  

The term refers to patients suffering from both COVID-19 and the common flu either simultaneously or back-to-back. 

Flurona is not a distinct disease, or even a combination of the two diseases, but simply the word used when a patient contracts both viruses at the same time. 

And while the term may be new, the bigger-picture confluence of both respiratory infections within the U.S. population has been happening for years. Health experts have been warning about the possibility of a “twindemic,” a scenario in which spikes in cases of COVID-19 and a simultaneous rough flu season overwhelm the country’s hospital systems, since early on in the pandemic. Instances of Americans testing positive for both COVID-19 and the flu have been reported as early as February of 2020, according to The Atlantic. 

Why is flurona happening now? 

Even before there was a buzzy name for it, flurona had been detected in different pockets around the world in recent weeks, including in Israel, Brazil, the Philippines, and Hungary. In the U.S., other instances of people who have tested positive for both the flu and COVID at the same time have been reported in Houston and South Florida. 

Health experts say the arrival of flurona cases may be attributed to the massive surge in Omicron cases, as the variant currently accounts for 95% of the country’s COVID infections.

Omicron’s rapid spread—the nation is averaging more than 585,000 new reported infections daily, a 254% increase compared with two weeks ago—comes during the heart of flu season in the U.S. and abroad.

And while last year’s flu season was the least harmful in more than a decade, experts warned Fortune in October that the possibility of the flu’s resurgence in the new year could be drastic, in part because fewer cases in the 2020–21 flu season caused “a reduced population immunity” that leaves the general public even more vulnerable to the flu this year. 

While it is too early to say if those experts were right, the CDC has reported an increase in common flu cases compared with last year in the U.S., most notably among young adults and children.

This resurgence, coupled with a surge in Omicron, means the potential for an increased number of flurona cases is possible. 

What are the symptoms? 

The common flu and COVID-19 have always had similar symptoms, including a cough, runny nose, sore throat, fever, headache, and fatigue. 

People may have varying levels of illness from either COVID-19 or influenza, ranging from no symptoms at all to severe and potentially fatal reactions, according to the World Health Organization. 

There is little data so far on specific symptoms reported by flurona patients. In Los Angeles, the teenager who tested positive for both illnesses reported “mild symptoms.” In Israel, the unvaccinated pregnant woman was reportedly “very sick,” although she has since recovered.

However, experts say catching both COVID and the flu at the same has the potential to produce a more severe reaction, but as is the case for either one, a patient’s severity of illness varies and depends largely on an individual’s immune system and, perhaps most important, whether or not they have been vaccinated against both diseases. 

How can I avoid it?

Health experts and public health officials are urging Americans to get vaccinated and boosted against COVID, which limits the possibility of a severe reaction. And as more and more Americans continue to contract the common flu, experts are emphasizing the importance of getting an annual flu shot.

In addition, public health officials stress a crucial way to avoid catching either disease is to wear a mask in public spaces, and continue to adhere to social distancing requirements.