Cold vs Flu vs Sinus Infection: Which One Do You Have?

Flu

Cold vs flu vs sinus infection: timing tells you more than any single symptom will. A cold builds over a day or two and peaks around day 3. The flu hits within hours, bringing fever, chills, and body aches with it. A sinus infection is usually a cold that’s still hanging around after 10 days, or one that’s started getting worse again.

Key Takeaways

• How fast it started is the best clue you have. A cold creeps in. The flu knocks you flat in an afternoon.
• A real fever, 100.4°F or higher, with body aches means a virus like the flu. Not a cold, and not allergies.
• Doctors call a sinus infection bacterial by the calendar: no improvement after 10 days, or a second slide after you’d started to feel better.
• Only 0.5% to 2% of colds turn into a bacterial sinus infection. The rest don’t need antibiotics.
• Green mucus proves nothing. Two published studies and all four doctors we spoke with say the same thing.
• The flu is the one you shouldn’t sit on, because antivirals work best in the first 48 hours. With a suspected sinus infection, waiting a week is usually the right move.

Cold vs Flu vs Sinus Infection at a Glance

All three come with a stuffy nose, a cough, and that heavy, run-down feeling, so it’s no wonder people mix them up. The differences are in how each one starts, how sick it makes you, and how long it hangs on.

	Cold	Flu	Sinus Infection
How it starts	Gradually, over 1 to 2 days	Suddenly, often within hours	Usually on the back of a cold or an allergy flare
Fever	Rare in adults, low grade if present	Common, often 100.4°F or higher for 3 to 4 days, though not everyone gets one	Uncommon. A high fever (102°F or more) with facial pain for 3 to 4 days is one pattern doctors watch for
Body aches and chills	Mild	Prominent	Uncommon
Fatigue	Mild	Severe, can last weeks	Mild to moderate
Runny or stuffy nose	Very common	Sometimes	Very common, with thick discharge
Facial pressure or pain	Mild, if any	Uncommon	The main clue, often worse when you bend forward
Sneezing and itching	Sneezing is common. Itchy eyes or throat suggest allergies instead	Uncommon	Uncommon
How long it lasts	Peaks day 2 to 3, mostly gone within about a week, cough can linger	Fever 3 to 4 days, tiredness and cough 1 to 2 weeks	Viral: improving by day 10. Bacterial: still there past day 10, or worse after improving
What causes it	More than 200 viruses, most often rhinoviruses	Influenza A or B viruses	Mostly viruses or allergies. Bacteria in a small minority
Is there a test?	No routine test	Yes, rapid and at-home tests	No. Diagnosed by timeline and exam
Antibiotics?	No	No. Antivirals can help if started within 2 days	Only if bacterial, and even then watchful waiting is an option

Sources: CDC, About Common Cold; CDC, Signs and Symptoms of Flu; AAO-HNS Clinical Practice Guideline: Adult Sinusitis Update, 2025; Acute Bacterial Rhinosinusitis review, PMC.

“There’s ways to tell, but it’s not a science,” says Dr. Robert Chandler, a family medicine physician who has practiced since 2008 and now sees patients exclusively through telehealth. Here’s how he and three of his DrHouse colleagues tell them apart.

Why Symptoms Alone Can Fool You

The CDC doesn’t sugarcoat it: colds and flu overlap so much that telling them apart on symptoms alone can be difficult or even impossible. That’s why flu tests exist.

The mistake Dr. Chandler sees most often is assuming this illness is a rerun of the last one. “I have a sore throat, it was strep throat, now it’s going to be strep throat again,” he says, playing the patient. “But just playing devil’s advocate, because that’s how we’re trained as doctors, this time could be different is what I have to tell people.”

Then there’s the problem of catching two things back to back. “Sometimes you can get more than one virus in a row. It can be hard sometimes to differentiate,” says Dr. Christin Barry, an emergency medicine physician with more than 10 years of experience in New York City. A cold that seems to drag on for three weeks is sometimes two colds in a row.

Is It a Cold?

A cold starts slowly and stays mild. Your throat felt scratchy last night, your nose is running this morning, and you can still get through the workday. That’s a cold.

The CDC counts more than 200 viruses that cause colds, and rhinoviruses are the most common in the US. Adults catch two to three a year on average. Symptoms peak within 2 to 3 days, and most colds are over in under a week. If an adult runs a fever with a cold, it’s usually a low one.

The sinus pressure you feel on day 3 or 4 belongs to the cold, too. Reviews of the sinusitis guidelines describe viral sinusitis as peaking in the first week and clearing by day 10. Keep that in mind, because pressure on day 4 is expected and pressure on day 12 is not.

Dr. Ragan Brackett, who is board certified in family medicine and obesity medicine and spent her early career in urgent care, gives patients a slightly longer window. “It’s been shown that if you have a viral upper respiratory illness, and that could be like a cold or a flu even, the average person’s symptoms should last between 10 and 14 days,” she says. “But if you have a history of allergies, your symptoms can be prolonged, two or three weeks even after those 10 to 14 days.”

So if you’re improving by day 7 to 10, even with a cough that won’t quit, your cold is doing what colds do.

Is It the Flu?

With the flu, you can usually name the hour it arrived. The CDC describes flu symptoms as coming on suddenly: fever or chills, cough, sore throat, aches, headache, and tiredness that can be severe. Its own cold-versus-flu chart lists onset as gradual for a cold and abrupt for the flu, with fever rare in colds and common in the flu, where it usually lasts 3 to 4 days.

Dr. Barry reaches for the thermometer to separate the flu from everything that imitates it. “A true fever for us is over 100.4,” she says. “Like 99.6, 99.8, sure, that’s okay, could still be allergies. But if it’s a real fever over 100.4, it’s probably something else.” The CDC’s definition lands on the same number: 100.4°F, or 38°C.

Two caveats. Not everyone with the flu runs a fever, and the CDC says so plainly. And the flu can lead to a sinus infection, which the CDC lists alongside ear infections as a moderate complication (pneumonia is the serious one). So “I had the flu and now my face hurts” is a real sequence, one illness leading into the next.

The flu is also the one condition here where waiting costs you something. “It’s also a double-edged sword, because with influenza and Tamiflu, now it’s the opposite,” Dr. Chandler says. “You have to get Tamiflu within two days. So now you’re telling the patient both sides. But if it’s body aches and chills and fever, or maybe somebody tested positive, usually patients will jump on that. Hey, my wife tested positive. Now I want to get treated.”

He’s describing CDC guidance: antiviral drugs work best when started within two days of the first symptoms, and they shorten the illness by about a day. Most healthy adults get through the flu without them.

For anyone at higher risk of complications, though, the CDC wants treatment started early. If that’s you, or if the fever and aches came on fast, a same-day online visit with a DrHouse doctor fits inside that window, and the doctor can prescribe oseltamivir (Tamiflu) when clinically appropriate.

Is It a Sinus Infection?

The rule doctors use here has nothing to do with the color of what comes out of your nose. It’s a date on the calendar.

The AAO-HNS adult sinusitis guideline, updated in 2025, tells clinicians to diagnose a bacterial sinus infection in only two situations. The first is when sinus symptoms (thick nasal discharge with congestion, facial pain or pressure, or both) haven’t improved at all 10 days after the cold began.

The second is when they get worse again within 10 days after they’d started to get better, a pattern the guideline calls double worsening. If you’ve been sick for under 10 days and you’re not getting worse, the guideline calls that viral, and viral sinusitis doesn’t benefit from antibiotics.

The bacterial group is small. Estimates cited in CMS quality measures built on the same guideline put the share of viral colds that go on to a bacterial sinus infection at 0.5% to 2%.

Dr. Vijaya Galic, a gynecologic oncologist by training who retired from surgery and now practices telehealth full time with DrHouse, explains what happens in those cases with a kitchen image. “Imagine you have left milk or coffee in a coffee cup and you’ve forgotten it for a couple of weeks.

You go look in it, and it’s all rotten at that point,” she says. “99% of sinusitis is going to be viral, allergic, or some combination. The excess mucus fills up the air spaces in the face, and if you don’t wash it out, it will just rot with what normally lives in your face.”

Her 99% is a clinician’s rounding of the same numbers, and it comes with a point people miss: “You have not caught the sinusitis from your coworker. You caught the virus, probably the rhinovirus or something, but the bacterial sinusitis that you develop a couple weeks later was from you not washing it out.” That’s why a saline rinse is the first thing she hands most sinus patients. “Nobody likes the neti pot,” she admits. “You have to do the neti pot.”

Dr. Chandler counts the same days from his side of the screen. “Usually after seven to 10 days, most of us providers will think about, well, now it’s at least something to talk about,” he says. Before that, in his words, “we think about over-the-counter stuff, which you can continue.” He’s candid about the gray zone, too. “If I tell you seven days is a sinus infection, what if you come on day six? What do we do?” Sometimes the answer is an antibiotic on day six. “You didn’t just wake up today and need an antibiotic.”

If any single symptom separates a sinus infection from a plain cold with some reliability, it’s pain or pressure in the face, around the cheeks, eyes, and forehead, often worse when you lean forward, along with congestion and thick discharge that won’t clear. A doctor can check for it on video.

“The one thing we’re missing is pushing on your face,” Dr. Chandler says. “But I could direct you, hey, can you push here? Does it hurt?” He’s found that works well enough. “It’s more of a visual, talking kind of diagnosis.”

The Mucus Color Myth

“People like to tell me all the colors of their mucus,” Dr. Chandler says. He breaks the news every day. “The green color of the mucus, I don’t know who started that, does not tell us anything. I hear that on a daily basis, multiple times a day.” Dr. Barry hears it from allergy patients, too. “Sometimes allergy mucus can still be yellow. Yellow does not automatically mean it’s bacterial or viral. That’s a myth.”

The research is on their side. In a peer-reviewed study of 241 adults with acute cough, Altiner and colleagues could prove a bacterial infection in only 12% of samples.

Yellow or green sputum did show up in 79% of the bacterial cases, but it also showed up in more than half of the non-bacterial ones (a specificity of 46%), which makes color barely better than a coin flip. A larger peer-reviewed study by Butler and colleagues followed 3,402 adults in 13 countries. People with discolored sputum were about three times as likely to walk out with an antibiotic, and they recovered no faster for it.

Which puts you back on the calendar: past day 10, or worse after getting better.

What About Allergies, COVID, and RSV?

Of everything in this article, allergies are what most often get mistaken for a stubborn cold, or for a sinus infection. Dr. Barry listens for three things. “If it’s sudden onset, you’re having fevers and flu symptoms, body aches. Allergies tend to be more like an itchy sensation in the throat or the nose,” she says. “If you’re not really having that symptom, then it’s more likely to be a virus. And also the timeline, if it’s a shorter duration, it’s more likely to be a virus. If it’s lasting more than two weeks, then it seems more likely to be allergies.”

Then the tiebreaker: “Allergies do not cause a fever. If you have a fever, you should suspect some other kind of infection, viral or bacterial.”

Dr. Brackett has learned not to ask patients whether they have allergies. “I don’t even ask them if they have allergies, because they’re going to say no,” she says. “So I ask, have you ever used Flonase or Claritin or Xyzal? And they’re like, yes. Well, you use the cure for allergies. So I assume that you do have them, even if you don’t recognize that you have them.” When one of those patients catches a cold, she tells them to treat more than the virus. “They need to start their allergy medicine too.”

The American Academy of Allergy, Asthma and Immunology draws the same line: most sinus infections begin with a virus like a cold, and people with allergies are more prone to sinus trouble in the first place.

COVID-19 and RSV can both pass for a bad cold or the flu, and the CDC says you can’t tell the flu from COVID-19 by symptoms alone. If you’ve got a fever, a cough, and aches in the middle of respiratory virus season, a combination test is the only way to know which one you have.

The Day-by-Day Timeline

Counting days is the most useful thing you can do to work out which of the three you have. Watch for two turning points: day 10, and any relapse after you’d started to improve.

Timeline	Cold	Flu	Sinus Infection
Day 1 to 2	Scratchy throat, sneezing, runny nose, building slowly	Sudden fever, chills, aches, exhaustion, often within hours	Not yet. This is still the cold or allergy flare that starts it
Day 3 to 5	Congestion peaks. Low-grade fever possible in adults	Fever typically lasts 3 to 4 days. Cough and fatigue heavy	Congestion and facial pressure can peak. This is viral sinusitis and it is expected
Day 6 to 9	Improving. Cough may linger	Fever usually gone. Tiredness and cough continue	Should be improving day by day
Day 10 and beyond	Mostly over. Two colds in a row can look like one long one	Cough and fatigue can persist into week two	No improvement at all by day 10 meets the guideline definition of a bacterial sinus infection
Got better, then worse	Unusual for a cold. Time to get checked	A CDC warning sign: fever or cough that improves, then returns or worsens	Double worsening, the second guideline definition of a bacterial sinus infection

Sources: CDC, About Common Cold; CDC, Signs and Symptoms of Flu; AAO-HNS Clinical Practice Guideline: Adult Sinusitis Update, 2025.

“A week is where you want to give them, to see if they’ll go away, because they might go away on day three, day four,” Dr. Chandler says of the infections patients suspect are bacterial. The flu is the exception. With the flu, the first two days matter most.

How a Doctor Tells Them Apart

Mostly by asking questions. “There’s not much on the physical exam that I’m really seeing,” Dr. Barry says. “It’s really more the history that’s telling me if it’s allergies or a virus.” Dr. Chandler describes the same routine. “There’s rubrics, there’s ways that us doctors go through it. Time, interval.”

With the flu, a test can settle it. “There’s tests,” Dr. Chandler says. “I don’t know the 100% accuracy of the home tests, but they’re more readily available now, the COVID test and the flu test. People will be like, here’s my flu test, I’m positive. So we generally would trust that. But subjectively from the symptoms, you can’t be 100%.”

The CDC’s numbers explain why he trusts a positive result and not a negative one. Rapid antigen flu tests catch only about 50% to 70% of true flu cases, so a negative strip during flu season doesn’t clear you. They’re more than 90% specific, though, so a positive one is rarely wrong. Molecular tests, the kind a clinic or lab runs, reach 90% to 95% sensitivity.

Dr. Galic takes the same view of the medical-grade home kits some of her patients keep in the cupboard: “I do trust the result if it’s positive.”

None of these doctors oversells what a video visit can do. “You went to urgent care and somebody listened to your lungs. That’s the difference,” Dr. Galic says. “I can’t do that. I can order a chest X-ray, and that’s it.”

Dr. Brackett is even blunter. “I can’t look in the ear. I can’t look down your throat and swab it. So sometimes I’m guessing, and sometimes I will tell them, if you’re continuing to have this problem, you need to go be evaluated physically in person. Stop using telehealth.” For the three conditions in this article, though, the history, the day count, a temperature, and a home test cover most of what a diagnosis needs.

Which Ones Need Medicine?

A cold needs time and comfort: rest, fluids, a saline rinse, and an over-the-counter pain reliever such as acetaminophen or ibuprofen for the aches or a fever. Antibiotics do nothing to a virus.

The flu has a treatment, and it’s a prescription with a clock on it. The CDC notes that antivirals like oseltamivir aren’t sold over the counter, work best within two days of symptoms, and shorten the illness by about a day. Most healthy adults recover without them. People at higher risk of complications, which includes anyone with asthma, diabetes, or heart disease, are the group the CDC says should be treated early, when clinically appropriate.

A sinus infection needs an antibiotic only if it’s bacterial, and even then the AAO-HNS guideline gives doctors two acceptable options: watchful waiting without antibiotics, or amoxicillin (with or without clavulanate) for 5 to 10 days. Most people with a bacterial sinus infection feel better within a week, and about 90% are cured or improved by day 15.

Doctors who push back on antibiotic requests aren’t being stubborn. The CDC estimates that at least 28% of antibiotics prescribed in outpatient settings are unnecessary. And when CDC researchers went through national visit data for a 2016 JAMA study, they concluded that about half of the antibiotics prescribed for acute respiratory conditions may have been unnecessary, and that sinusitis produced more antibiotic prescriptions than any other single diagnosis.

Dr. Chandler chooses his words carefully when a patient asks for antibiotics for a virus. “I feel that your symptoms are from a virus. Or I feel that you don’t have a bacterial infection, so antibiotics are not indicated,” he tells them. “Bacteria and viruses are different.” He’s just as direct about leftover pills. “You’re giving the bug a vaccine if you’re taking antibiotics incorrectly. If you take it for a day, it’s stop and start. If you take two because you had two left over, you’re vaccinating the bug. You’re giving them the power to beat our antibiotics, and that can be very dangerous.”

Dr. Galic had that conversation the day we spoke. “I had a 23-year-old today with a cold, and he wants a Z-Pak,” she says. “You’re a viral cold. You’re going to take these drugs that I’m giving you on my list, you’re going to be fine. You definitely don’t need to be messing with the azithromycin. Because when you actually get something I have to treat it with, it’s going to be resistant.” She hears the other half of that story constantly. “I get a lot of patients who are like, I got antibiotics for this and they didn’t work. And I’m like, well, because you have a virus.”

Dr. Brackett holds herself to the same rule at home. “I don’t give my own self antibiotics. I don’t give it to my kids,” she says. “Usually when I think that they’re sick, I only treat when I think it’s appropriate.”

When to See a Doctor

Most colds, most cases of the flu, and most sinus infections get better on their own. Here’s when a visit changes the outcome:

• A sudden fever of 100.4°F or higher with body aches within the last 48 hours, especially in flu season or after a known exposure. That’s the antiviral window.
• You’re at higher risk from the flu (pregnancy, asthma, diabetes, heart disease, age 65 or older) and you have flu-like symptoms, at any point.
• Cold or sinus symptoms that haven’t improved at all after 10 days.
• Symptoms that got better and then got worse again, with a returning fever, worse congestion, or new facial pain.
• Two weeks of allergy medicine and no change.
• Facial swelling, a severe headache, vision changes, or a stiff neck along with sinus symptoms.

Skip the video visit and go to the emergency room for the CDC’s flu warning signs: trouble breathing, persistent pain or pressure in the chest or abdomen, persistent dizziness or confusion, seizures, not urinating, severe muscle pain or weakness, or a fever or cough that eases and then comes back worse.

For everything on the first list, a telehealth visit is usually enough to work out which of the three you have and whether you need a prescription at all. DrHouse doctors are available around the clock for cold and flu visits, and for sinus infections, too. And as Dr. Brackett said, if you need to be seen in person, they’ll tell you.

Frequently Asked Questions

Can a Cold Turn Into a Sinus Infection?

Yes, though rarely into a bacterial one. Nearly every cold inflames the sinuses, so pressure and congestion peak around days 3 to 6. Only an estimated 0.5% to 2% of colds go on to a bacterial infection. You’ll know it has happened if your symptoms haven’t improved by day 10, or if you clearly slide backward after starting to recover.

Can a Cold Turn Into the Flu?

No. Different viruses cause them, so one can’t become the other. What does happen is catching a second virus while you’re still getting over the first. “Sometimes you can get more than one virus in a row,” Dr. Barry says, and that’s usually what people mean when they say their cold “turned into” the flu.

How Can I Tell if I Have the Flu or a Cold Without a Test?

Speed and severity. The flu arrives within hours, with a fever, chills, and aches that keep you in bed. A cold builds over a day or two, stays mostly in your nose and throat, and rarely gives an adult a real fever. The CDC cautions that symptoms alone can’t always separate the two, so if the answer would change your treatment, take a test. Trust a positive result. Don’t fully trust a negative one.

Are Colds More Contagious Than the Flu?

There’s no clean ranking. Both spread the same way, through droplets from coughing and sneezing and through hands and surfaces. What the CDC can say is when the flu spreads most: in the first 3 days of illness. Staying home while you have a fever protects the people around you, whichever virus you’ve got.

Will a Sinus Infection Go Away on Its Own?

Usually, yes. Viral sinusitis clears up with the cold, typically by day 10. Even bacterial sinus infections often resolve without antibiotics, which is why the AAO-HNS guideline allows watchful waiting as a first step. It notes that about 90% of patients with a bacterial sinus infection are cured or improved by day 15. Saline rinses, steam, and time do most of the work.

Does Green Mucus Mean I Need Antibiotics?

No. Two peer-reviewed studies found that yellow or green mucus is a weak sign of bacterial infection, and that antibiotics didn’t speed recovery in people who had it. Color tells you how hard your immune system is working. It says nothing about what it’s fighting. Doctors decide based on the timeline: past 10 days, or worse after improving.

Can You Have the Flu Without a Fever?

Yes. The CDC states that not everyone with flu will have a fever. Sudden exhaustion, chills, aches, and a cough during flu season are reason enough to suspect the flu and consider a test, especially if you’re at higher risk of complications.

Sources

• Centers for Disease Control and Prevention. About Common Cold. https://www.cdc.gov/common-cold/about/index.html
• Centers for Disease Control and Prevention. Cold Versus Flu. https://www.cdc.gov/flu/about/coldflu.html
• Centers for Disease Control and Prevention. Signs and Symptoms of Flu. https://www.cdc.gov/flu/signs-symptoms/index.html
• Centers for Disease Control and Prevention. Treatment of Flu. https://www.cdc.gov/flu/treatment/index.html
• Centers for Disease Control and Prevention. Flu: What To Do If You Get Sick. https://www.cdc.gov/flu/takingcare/index.html
• Centers for Disease Control and Prevention. Overview of Influenza Testing Methods. https://www.cdc.gov/flu/hcp/testing-methods/index.html
• Centers for Disease Control and Prevention. Definitions of Signs, Symptoms, and Conditions of Ill Travelers. https://www.cdc.gov/port-health/php/definitions-symptoms-reportable-illness/index.html
• Centers for Disease Control and Prevention. Antibiotic Use in the United States. https://www.cdc.gov/antibiotic-use/hcp/data-research/antibiotic-prescribing.html
• Payne SC, et al. Clinical Practice Guideline: Adult Sinusitis Update. Otolaryngology–Head and Neck Surgery, 2025. https://aao-hnsfjournals.onlinelibrary.wiley.com/doi/10.1002/ohn.1344
• Acute Bacterial Rhinosinusitis. PubMed Central review summarizing AAO-HNS and IDSA criteria. https://pmc.ncbi.nlm.nih.gov/articles/PMC7122468/
• Centers for Medicare and Medicaid Services. Quality ID #331: Adult Sinusitis: Antibiotic Prescribed for Acute Viral Sinusitis (Overuse). https://qpp.cms.gov/docs/QPP_quality_measure_specifications/CQM-Measures/2021_Measure_331_MIPSCQM.pdf
• Altiner A, et al. Sputum colour for diagnosis of a bacterial infection in patients with acute cough. Scandinavian Journal of Primary Health Care, 2009. https://doi.org/10.1080/02813430902759663
• Butler CC, et al. Antibiotic prescribing for discoloured sputum in acute cough/lower respiratory tract infection. European Respiratory Journal, 2011. https://doi.org/10.1183/09031936.00133910
• Fleming-Dutra KE, et al. Prevalence of Inappropriate Antibiotic Prescriptions Among US Ambulatory Care Visits, 2010-2011. JAMA, 2016. https://jamanetwork.com/journals/jama/fullarticle/2518263
• American Academy of Allergy, Asthma and Immunology. Colds, Allergies and Sinusitis: How to Tell the Difference. https://www.aaaai.org/tools-for-the-public/conditions-library/allergies/colds-allergies-sinusitis