Have a runny nose? Fever? Body aches? During flu season, it's natural to think you've been hit with the flu, but how do you know it's not just a nasty cold?

Cold and flu are both respiratory illnesses that share some of the same symptoms, but they are caused by different viruses. It's important to know the difference so you can get the right treatment and feel better.

Colds are generally more mild than the flu, while flu symptoms come on more suddenly and typically are more intense, according to the Centers for Disease Control and Prevention.

Cold symptoms

With a cold, you're likely to start with a sore throat for the first day or two, then have things move into your nose, reports WebMD. You'll have a runny nose and congestion, then a cough. In the beginning, the secretions from your nose will be watery, then get thicker and darker. (Just because it's dark doesn't mean it's a sinus infection.)

It's rare for an adult with a cold to have a fever. But not so for a child; a child is more likely to have a fever with a cold. It's also uncommon to have body aches, headache or chills.

You're contagious for about the first three days you have cold symptoms, although your symptoms typically last for about a week. If symptoms aren't improving after a week, check with your doctor to see if you've developed a bacterial infection, which means you need antibiotics. You may also be suffering from allergies.

There's no cure for the cold. Treat symptoms as necessary with pain medication, cough medicine or decongestant nasal sprays, says the Mayo Clinic. You can also try soothing lifestyle and home remedies like drinking lots of fluids, getting plenty of rest, eating chicken soup and soothing your cough naturally.

Flu symptoms

Flu symptoms usually are more severe and come on more rapidly than cold symptoms. That's why people often liken the flu to "being hit by a truck."

Flu symptoms include fever, chills, cough, sort throat, headache, muscle aches and soreness, and congestion. If you have the flu, you might also have sneezing and a runny or stuffy nose.

Most symptoms start to improve over the first two to five days, according to WebMD, but it's not unusual to feel poorly for a week or longer. Fatigue and weakness can sometimes last for weeks after other symptoms are gone.

According to the CDC, you're likely able to infect other people beginning one day before symptoms develop and you are contagious up to 5 to 7 days after becoming sick.

Flu can sometimes lead to more serious complications, such as pneumonia and hospitalization, especially in people who are young, elderly or have other health problems. Contact your doctor if you notice shortness of breath, or a fever that returns after having been gone for a few days.

There's no cure for the flu, but if you see your doctor as soon as symptoms start, she may prescribe an antiviral medication, such as oseltamivir (Tamiflu) or zanamivir (Relenza).

Another new antiviral drug will soon be available at pharmacies. Xofluza (baloxavir marboxil) is the first antiviral flu medication approved by the U.S. Food and Drug Administration in almost 20 years. The oral drug is administered in a single dose for patients who are 12 years and older.

If taken early, these drugs may shorten your illness by a day or so, lessen the severity of symptoms, and help prevent serious complications. Otherwise, as with the cold, treat your symptoms as they pop up.

The flu is less common that we think

two children sick on the bed Children are likely to get the flu about every other year. (Photo: YAKOBCHUK VIACHESLAV/Shutterstock)

Although it seems like the flu is everywhere, according to a 2015 study published in the journal PLoS Biology, the flu is much less common than many people realize.

Researchers from the Imperial College of London found that the average adult only contracts the flu about twice every decade.

Researchers analyzed blood samples from volunteers in southern China to evaluate their levels of antibodies against nine different flu strains that circulated there between 1968 and 2009. They found that, on average, children get the flu every other year, but flu infections become less frequent as people get into their 20s and 30s. After age 30, people tend to get the flu about every five years.

Children are considered "super-spreaders" of the flu because they interact with more people more often and also may be less diligent about hygiene habits like hand washing and covering their mouths when they cough. They also have not yet built up an immunity to many flu strains simply because they haven't been exposed to them.

Maybe it's an adenovirus

It could be the cold or the flu. Or it could be an adenovirus.

Adenoviruses can feel like the flu with symptoms that include fever, sore throat and bronchitis, according to the CDC. They can also cause non-cold-like symptoms such as diarrhea, bladder infections and pink eye (conjunctivitis). Unlike the cold and flu, which are more common in the winter months, adenoviruses can occur any time of year.

Although most adenovirus-caused infections aren't severe, people with weakened immune systems or the very old or young can develop complications, including pneumonia and rare neurological symptoms such as encephalitis — brain inflammation — and meningitis.

Adenoviruses are difficult to diagnose, Dr. William Schaffner, an infectious disease specialist at Vanderbilt University, told CNN, since they're not included in standard tests for viruses.

Bur Schaffner doesn't think people need to be worried about adenoviruses. "They cause principally a whole bunch of minor troublesome infections spread by children, often from children to adults," he said. "But they're not nearly as serious as influenza."

Editor's note: This story was originally published in March 2015 and has been updated with new information.

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

Do I have a cold or the flu?
You will feel horrible with either, but there are some differences between a cold and the flu.