The second flu season of the pandemic is off and running, and every cough and sniffle will carry an extra worry: possible COVID-19 infection.

With influenza and the usual cold-weather respiratory infections in circulation, along with COVID-19, it's not always easy to sort out what's ailing you if you do get sick.

We talked to UC San Francisco emergency care physician Jahan Fahimi, M.D., and infectious disease specialist Peter Chin Hong, M.D., about the differences between flu and COVID-19, when to get a test, and why you shouldn't try to diagnose yourself.

Note: If you think you're experiencing a life-threatening or severe condition, call 911 or go directly to the nearest emergency department.

Is it possible to tell the difference between flu symptoms and COVID-19 symptoms?

"I think it's tough because the flu and COVID-19 can have a variety of overlapping symptoms," said Fahimi. Those can include fever, chills and body aches, upper respiratory symptoms like runny nose and sore throat, lower respiratory symptoms like cough and pneumonia, and gastrointestinal symptoms like nausea, vomiting and diarrhea.

"While certain symptoms are slightly more associated with one virus than the other, there's enough overlap that there's uncertainty," cautioned Fahimi. "So we wouldn't use the presence or absence of those symptoms to rule in or out either illness."

Typical flu symptoms are relatively consistent: fever, cough and muscle aches. These are also common in COVID-19, but COVID-19 symptoms vary more wildly than those of the flu, from no symptoms at all to deadly pneumonia and myriad cardiovascular and neurological issues, said Chin-Hong.

"Influenza is engineered to cause disease, so once you get it, you're going to have symptoms and be stuck in bed," said Chin-Hong. "COVID depends on what soil it lands on. Once you get COVID, you can have a wide range of consequences."

Does COVID-19 have any telltale symptoms?

One sign that does seem more specific to COVID-19 is the sudden loss of smell or taste. Initially underappreciated as a symptom, it's now thought to be common.

"That's one symptom that I would be particularly concerned about for COVID-19," said Fahimi.

Another sign that can point to COVID-19 is low oxygen saturation, which can cause shortness of breath, although it often goes unnoticed without a pulse oximetry measurement.

Chin-Hong noted that some symptoms in isolation – such as allergy-like symptoms, like runny nose and water eyes, or diarrhea by itself – are less likely to be a sign of COVID-19, because they usually occur in conjunction with other symptoms of the disease.

Children under 10 are less likely to be infected with and show symptoms of COVID-19, so if they have flu-like symptoms, they're more likely to have the flu, he added.

But these are generalizations. Thanks to the variable nature of COVID-19, the only way to know whether you're infected is to be tested.

"I can't give you a magic formula," said Chin-Hong. "At the end of the day, it's really testing that's going to tell you."

What should you do if you feel sick?

If you feel unwell and have any of these symptoms, it's best to self-quarantine at home (including avoiding contact with other household members) – and to get a COVID-19 test. Testing is free, nationwide, and many testing sites take appointments without a doctor's referral.

"Out of an abundance of caution, we have to try to catch all these cases where we can," said Fahimi.

The benefit of knowing you have COVID-19 is that you can take precautions to prevent spreading the disease, contact tracers can alert people you've been around recently, and doctors can follow up in case your condition gets worse.

"We may be able to preemptively get someone into care a little bit sooner, which could impact the course of their disease," said Fahimi. 

Flu tests are also widely available, but they generally require a visit to the doctor's office or urgent care. When diagnosed early, the flu can be treated with the antiviral Tamiflu.

Even if you test negative for both COVID-19 and the flu, it's best to self-quarantine for three days after your symptoms disappear, said Fahimi. There's a small chance that a negative test result will turn out to be wrong. Moreover, many other respiratory illnesses, from the common cold to croup, circulate during flu season, and spreading them would also spread uncertainty.

"Everyone's focused on the flu and COVID-19, but there's a whole host of other stuff going around, which can present very similarly," said Chin-Hong. 

When should people seek medical attention right away?

If you experience shortness of breath, seek medical care right away, said Chin-Hong. You may notice, for example, that it's harder to catch your breath walking up the stairs. That could be a sign that your oxygen levels are low, possibly due to COVID-19.

If you're older, immunocompromised, or have other health conditions that make you more vulnerable to COVID-19, contact your doctor "sooner than later" if you have any symptoms, said Chin-Hong.

Do COVID-19 and the flu spread differently?

Both flu and COVID-19 are spread mainly by respiratory droplets launched through coughs and sneezes. But COVID-19 can also be transmitted more stealthily, said Chin-Hong – through smaller droplets ("aerosols") emitted through talking or singing that can linger in the air.

Also, people with COVID-19 can transmit the virus up to two days before they show symptoms or even if they never develop symptoms. Many people with COVID-19 infections are "asymptomatic," meaning they have no symptoms. In contrast, said Chin-Hong, most people with the flu are well aware that they have it.

"That's why COVID-19 is so insidious. People don't even know they are infected, and they can spread it," he said.

Will we have enough tests?

The most accurate tests for COVID-19 and the flu are both PCR tests that require some of the same chemicals to process. This raises concerns that surges in both infections this winter could strain testing capacity. 

You can help prevent a shortage of tests, said Chin-Hong, by getting a flu shot. Although flu shots won't prevent all cases of the flu, they do reduce the number of people with conflicting symptoms who may need tests and "take a segment of the population out of that equation."

The best way to avoid the "What do I have?" question this winter is to get the flu shot and keep wearing masks, washing hands and maintaining social distance.

"We want to reduce the number of people getting these symptoms and having to make these decisions in the first place, because they're going to be difficult," said Chin-Hong. "My advice is: Don't even be in this position."