Ah, January. The season of new beginnings, icy winds, and respiratory infections. With a “quad-demic” of diseases circulating the country—flu, COVID, respiratory syncytial virus (RSV), and norovirus, a gastrointestinal illness—it can be difficult to be sure which is making you sick. Throw in the common cold, and you’ve got a symphony of similar symptoms.

The flu is having a moment. Nationwide test positivity was at a season-high 18.7% the week ended Dec. 28, compared to 2.1% six weeks earlier, according to the Centers for Disease Control and Prevention (CDC). What’s more, influenza-like illness activity was elevated in all 10 regions the agency surveils. So how can you be sure you have the flu?

As with COVID, testing is the only way to confirm the presence of the influenza virus in your body. However, the CDC notes that your doctor may diagnose you based on your symptoms alone. The more accurate flu tests—reverse transcription polymerase chain reaction (RT-PCR), viral culture, and immunofluorescence assays—require a health care provider to swipe the back of your throat or the inside of your nose, then send the swab to a specialized lab.

While less accurate, rapid influenza diagnostic tests (RIDTs) and rapid molecular assays are more accessible. You can purchase an RIDT over the counter at a number of pharmacies, big-box stores, and online retailers. If you’re unable to complete such testing on your own, urgent care centers, medical testing facilities, and other clinics can do it for you and provide results within 10 to 20 minutes, depending on the type of test.

Some over-the-counter and laboratory tests can differentiate between COVID, influenza A, influenza B, and/or RSV in a single sample. It’s also possible to have flu and another respiratory illness at the same time, though the CDC says it’s unclear how common this is.

What are the symptoms of the flu?

Knowing which respiratory illness you do or don’t have not only assists public health officials in tracking disease spread, but also helps your health care provider determine the best course of treatment for you. You might consider flu testing if you have the following symptoms, as noted by the CDC:

• Cough

• Fatigue

• Fever or feeling feverish/chills (not everyone will have a fever)

• Headaches

• Muscle or body aches

• Runny or stuffy nose

• Sore throat

• Vomiting and diarrhea (more common in children)

Which flu symptoms require emergency care?

Most people who catch the flu have only the above mild symptoms and recover on their own without the use of antiviral prescription medication. Still, flu can be serious and even deadly for older adults, young children, and people with certain conditions including asthma, obesity, and heart disease.

“Every year in this country we have thousands and thousands of patients who are hospitalized and die with influenza, and they are particularly older people or people with medical illness,” Dr. Michael Ben-Aderet, associate medical director of hospital epidemiology at Cedars-Sinai Medical Center in Los Angeles, previously told Fortune. “It is beneficial to those people and everyone else that we get the flu vaccine.”

If you or a person in your care shows any of the following signs of flu complications, the CDC recommends seeking emergency medical care:

• Adults

o Difficulty breathing or shortness of breath

o Fever or cough that improves but then returns or worsens

o Not urinating

o Persistent dizziness, confusion, inability to arouse

o Persistent pain or pressure in the chest or abdomen

o Seizures

o Severe muscle pain

o Severe weakness or unsteadiness

o Worsening of chronic medical conditions

• Children

o Any fever in children younger than 12 weeks

o Bluish lips or face

o Chest pain

o Dehydration (no urine for 8 hours, dry mouth, no tears when crying)

o Fast breathing or trouble breathing

o Fever above 104 degrees that’s not controlled by fever-reducing medicine

o Fever or cough that improves but then returns or worsens

o Not alert or interacting when awake

o Ribs pulling in with each breath

o Seizures

o Severe muscle pain (child refuses to walk)

o Worsening of chronic medical conditions

Is January too late to get the flu shot?

In a word, no, says Dr. Robert Hopkins Jr., medical director of the National Foundation for Infectious Diseases.

“It’s not too late,” Hopkins told Fortune in December. “It is not a bad time when we’ve got risk in front of us. And I would certainly prefer that people were vaccinated earlier, but I’m not going to make perfection the enemy of the good.”

And while the 2024–25 seasonal flu vaccine doesn’t protect against H5N1 bird flu, the more people who get the vaccine, the lower the chances of the current bird flu outbreak becoming a pandemic, the CDC says. That’s because it’s possible for the two flu types to exchange genetic material, forming a new flu virus that could spread among humans.

Yet vaccination coverage this season has been lacking. Less than half of adults (42.7%) had gotten their annual flu shot as of the week ended Dec. 28, as had about the same proportion of children (41.9%).