Coronavirus and the flu have similar symptoms. So, how are they different?

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Is it the flu, a cold or the new coronavirus? Patients and doctors alike are parsing signs of illness to figure out who needs what tests or care and how worried they should be.

“You have three different major viruses floating around at the same time,” causing somewhat similar symptoms—but different levels of concern, said Dr. Gary LeRoy, president of the American Academy of Family Physicians.

So what’s the biggest danger? And why are we responding to them so differently?

Familiar foe

COVID-19, the disease caused by the new coronavirus, is a flu-like illness that has killed a small fraction of the number of people that the flu kills every year. Through the first four months of the outbreak, coronavirus has killed about 4,300 people. Flu kills 290,000 to 650,000 every year around the world, according to the World Health Organization.

To some, that comparison seems comforting because flu is such a familiar foe. President Donald Trump regularly brings it up, noting in a tweet how many more Americans die from flu and adding, “Nothing is shut down, life & the economy go on … Think about that!”

But to public health experts, the huge number of flu deaths is exactly why extraordinary steps should be taken to try to prevent the new coronavirus from spreading widely.

The flu’s annual return can’t be stopped because it’s already so embedded in the population. There is still a chance COVID-19 cases can be limited or spread slowed while treatments are developed.

How deadly are coronavirus and the flu?

Flu kills about 0.1% of those it infects, but that’s still hundreds of thousands of people each year because it infects millions.

Researchers are still trying to understand just how deadly the new coronavirus is. The mortality rate from infection with the virus isn’t known yet because the cases caught in an early part of an outbreak are often the most severe, people with mild or no symptoms aren’t being tested, and sometimes overwhelmed hospitals struggle to care for the sickest patients. Various reports have estimated the fatality rate from less than 1% to as high as 4% among cases diagnosed so far, depending on location.

Most people infected by the new coronavirus develop mild or moderate symptoms and recover after about two weeks.

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So what do I have: Coronavirus or the flu?

Flu, cold and coronavirus often share certain symptoms, but differences in intensity and how they appear can offer clues to which one is causing the misery. Doctors can test for the flu and get results within a day, but coronavirus testing is still limited by availability in the United States.

Colds are often suspected because adults get about two on average each year, said LeRoy, a family medicine doctor and associate dean at Wright State University in Dayton, Ohio.

“The common cold just starts out with a sore or scratchy throat, cough, runny nose, stuffy nose” and any fever is usually mild, he said.

Flu symptoms are more intense and usually come on suddenly, the Yale New Haven Health System advises. They can include a high fever (over 100.5 degrees), extreme exhaustion, muscle or body aches, a dry cough and chills.

“It really hits you like a bus,” and people may start a day well but feel terrible by afternoon, LeRoy said.

Flu symptoms can include a runny or stuffy nose, headaches and possibly vomiting or diarrhea, though the latter two are more common in children than adults, the U.S. Centers for Disease Control and Prevention says.

Symptoms of COVID-19 may appear more slowly. They usually include fever, a dry cough and noticeable shortness of breath, according to the World Health Organization. A minority of cases develop pneumonia, and the disease is especially worrisome for the elderly and those with other medical problems such as high blood pressure, obesity, diabetes or heart conditions.

One study of hospitalized patients in China found that about half did not have a fever when they were admitted but nearly all developed one.

What to do if you’re sick

Don’t go straight to your doctor’s office—that just risks making more people sick, officials urge. Call ahead, and ask if you need to be seen and where.

Fever, cough and noticeable shortness of breath—”if you have those three components, especially if it’s associated with some recent travel or someone you know who’s been exposed to COVID-19, those things should prompt you to call for medical attention,” LeRoy said.

“Mildly ill patients should be encouraged to stay home,” the CDC’s Dr. Sue Gerber told doctors on a conference call last week. People having difficulty breathing should seek care, and older people or those with other conditions should contact their doctors early in the course of illness, she said.

How to prevent getting sick

To protect yourself, wash your hands well and often, keep them away from your face, and avoid crowds and standing close to people.

There’s one big difference between flu and coronavirus: A vaccine exists to help prevent the flu and it’s not too late to get it. It won’t protect you from catching the coronavirus, but may put you in a better position to fight it.

“You don’t want to have a compromised immune system if you were to encounter coronavirus,” LeRoy said.

More coronavirus coverage from Fortune:

—How coronavirus is affecting the global concert industry
Politicians around the world are going into quarantine
—Some of the most extreme ways companies are combating coronavirus
—How Europe is adapting to the coronavirus outbreak
—What Xi Jinping’s visit to Wuhan says about China’s coronavirus recovery
Conferences go online amid coronavirus fears—minus the hallway schmoozing
—Coronavirus may not be all bad for tech. Consider the “stay at home” stocks

Subscribe to Fortune’s Outbreak newsletter for a daily roundup of stories on the coronavirus outbreak and its impact on global business.

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