8 Illnesses That Cause Flu-Like Symptoms That Aren’t the Flu

Considering that the flu can be highly contagious, it's tempting to worry that every cough, muscle ache, and hint of a fever is a sign you caught the flu.

That's understandable, given that there are a ton of other illnesses that bring on copycat flu-like symptoms, like fever, cough, runny nose, and headaches, among others.

"Any overwhelming infection that stimulates our immune system can [produce] some of the same symptoms," Cindy Weston, DNP, RN, FNP-BC, an associate professor at the Texas A&M School of Nursing in Bryan, Texas, told Health.

In other words, "most flu just feels like a regular cold," Joseph Khabbaza, MD, a pulmonologist with the Cleveland Clinic in Independence, Ohio, told Health. Additionally, flu vaccines help reduce the severity of symptoms in those who are vaccinated.

The only way to know for sure you have the flu is to get tested. But sometimes, there are subtle clues to help you distinguish between the flu and something else. Getting tested can also help distinguish between the flu and COVID-19.

Here's what you need to know about a few of the many conditions that can cause flu-like symptoms but aren't the flu.

Colds

Both colds and influenza are viral illnesses, they both tend to occur in the same seasons, and they have many overlapping symptoms, like a sore throat and a stuffy nose. The main difference is how quickly the symptoms come on.

"A cold typically gradually progresses symptom by symptom over [several] days," Keri Peterson, MD, an internist based in New York, told Health. "With the flu, the constellation of symptoms of high fever, cough, muscle ache, and severe lethargy comes on in 24 to 48 hours."

And even though so many symptoms overlap, colds typically don't come with chest pain or body aches, which are more characteristic of the flu, added Dr. Peterson.

Strep Throat

The flu and strep throat share many symptoms, but there are two you may find in the flu but never in strep: Cough and nasal congestion.

Strep throat may also bring swollen lymph nodes (bean-shaped structures that are part of the immune system), swollen tonsils (soft tissue masses at the back of the throat), tiny red spots on the roof of the mouth, or white blotches on the tonsils known as exudates. None of those symptoms are typical of the flu.

If a healthcare provider suspects strep, they will probably swab your throat and test for the bacteria. If the test comes back positive, you'll likely get antibiotics, which can usually clear up the symptoms quickly.

Pneumonia

Pneumonia can come separately from the flu, or it can be a secondary complication of getting sick. You may even look like you're over the flu and become stricken with another infection. 

"People are getting the flu and maybe even riding it out, and a week or so later, they're coming in with pneumonia," noted Weston.

Pneumonia that comes with or after the flu can be caused by the flu virus itself or by co-infections of the flu virus and bacteria. Bacterial pneumonia is very serious and can be treated with antibiotics.

With that type of pneumonia, "the cough is pretty persistent and unrelenting and often associated with chest pain," explained Weston. "The fever could be low grade or higher. A lot of times there's no appetite with pneumonia, and there can be some body aches." A pneumonia cough also has mucus in it.

Viral pneumonia is typically milder than the bacterial kind. You may also have some congestion, coughing, and fatigue.

Healthcare providers can listen for telltale signs of pneumonia by putting a stethoscope on your chest, said Dr. Peterson.

Mononucleosis

Mononucleosis, also called the "kissing disease," or just mono, passes through saliva (along with coughing, sneezing, and sharing utensils).

Mono is usually caused by the Epstein-Barr virus and tends to hit adolescents and young adults more than other age groups.

Symptoms often come on slowly, but they can mimic the flu. You might feel really, really tired, spike a fever, or have a sore throat and body aches. But other symptoms can help differentiate mono from the flu, including a swollen liver or spleen.

Mono also drags on longer than the flu, often lasting two to four weeks, but sometimes even months longer.

Meningitis

Meningitis is inflammation of the membranes (meninges) that cover the brain and spinal cord.

Like pneumonia, meningitis can be caused by a viral or bacterial infection. Viral meningitis is more common and milder, but the symptoms of both are similar and look a lot like the flu: headache, fever, and fatigue.

Meningitis also comes with a stiff neck and sensitivity to bright light.

Viral meningitis is like colds and the flu in that most people recover on their own in a week or so. Bacterial meningitis, however, can cause brain damage and even death if it's not treated promptly with antibiotics.

Bronchitis

Acute bronchitis not only has cold- and flu-like symptoms, but it's also even caused by many of the same viruses.

"Bronchitis has a lot of overlap–productive cough with mucus, lethargy, and a sore throat," explained Dr. Peterson. The main difference is that bronchitis doesn't come with a high fever.

Bronchitis symptoms also tend to center on your chest and throat instead of the full-body aches common with the flu, added Dr. Peterson. The nagging cough of an acute bout of bronchitis can last up to three weeks, which is longer than a cough from the flu.

There's no test for bronchitis like there is for the flu, so healthcare providers usually diagnose it by asking about symptoms and examining you. Bronchitis treatment consists of rest, drinking lots of fluids, and taking meds that can relieve symptoms.

Respiratory Syncytial Virus (RSV)

Respiratory syncytial virus (RSV) has symptoms that can also be mistaken for the flu (or a cold). 

"It can cause runny nose and cough," Afif El-Hasan, MD, a pediatrician with Kaiser Permanente in Orange County, Calif, told Health.

Unlike the flu, though, RSV symptoms usually appear gradually. They typically go away on their own, as well. You just need to drink plenty of fluids and rest.

Similar symptoms come from infection with what are called human parainfluenza viruses.

"They are like the flu, but they're not as bad," noted Dr. El-Hasan.

Human Immunodeficiency Virus (HIV)

About two to four weeks after becoming infected with human immunodeficiency virus (HIV), most people experience flu-like symptoms. Those symptoms may include fever, chills, rashes, night sweats, or muscle aches. They may last for a few days or linger for several weeks.

Although there are many other reasons that you may have flu-like symptoms, if you think that you may have been exposed to HIV, it's important to get tested. The CDC also recommends that all individuals between the ages of 13 and 64 get tested for HIV at some point.

When Should You Contact A Healthcare Provider?

"If you suspect you have the flu, then you should see [a healthcare provider] within 48 hours because [antiviral] medicine has to be taken very quickly," said Dr. Peterson. "Err on the side of caution."

Dr. Khabbaza added that if you're unable to go about your normal routine, you should consider heading to a healthcare provider. Also, if certain symptoms get worse, that could be an indication of the flu or another serious condition, not just a chest cold, and you, again, should consider getting checked out as soon as possible.

For example, if your body aches progress "to the point you can barely move around," or if your symptoms are causing difficulty breathing, it's definitely time to head to a healthcare provider, said Dr. Khabbaza.

Shortness of breath or chest pain, in particular, can indicate many medical emergencies, such as a pulmonary embolism (a blood clot that blocks an artery supplying blood flow to the lungs), that require prompt treatment.