The flu is a highly contagious respiratory infection caused by the influenza virus. The virus causes seasonal epidemics of respiratory illness that occur during the fall and winter months.

Despite the seasonality of influenza activity, many people experience flu-like symptoms during the summer. Although the Centers for Disease Control and Prevention detect influenza viruses year-round, these symptoms may not be due to influenza infection.

Flu season is when influenza activity is highest. The incidence of influenza infection typically begins to increase in October and peaks in the winter months of December, January, or February.

It’s believed that the seasonal nature of influenza may be due to the colder, drier climate that’s present during the winter months. During this time, the virus may be more stable. A study in a guinea pig model supports this idea, finding that influenza viruses are transmitted more effectively between animals at a low humidity and low temperature.

Another factor that may contribute to influenza peaking in the winter could be the fact that people spend more time indoors. This makes them more likely to share an enclosed space with infected individuals. Additionally, lower levels of vitamin D due to less exposure to sunlight could possibly contribute to an increased susceptibility to infection.

When you have the flu, symptoms typically come on suddenly. They can include:

  • fever
  • chills
  • coughing or sneezing
  • headache
  • body aches and pains
  • runny or congested nose
  • sore throat
  • fatigue

The symptoms of flu are also common symptoms of other illnesses. If you’re experiencing flu-like symptoms during the warmer months of the year, they may be due to another illness or condition other than the flu.

Some possible illnesses that may give you flu-like symptoms in the summertime include:

Common cold

The common cold is another respiratory infection caused by a variety of viruses.

There’s a lot of overlap between the symptoms of the common cold and those of the flu, such as a runny nose or congestion, coughing or sneezing, and sore throat.

However, unlike the flu, the symptoms of the common cold develop gradually and are most often less severe. There are other differences between a cold and flu as well.

Gastroenteritis

Although gastroenteritis is often referred to as the “stomach flu,” it’s not related to influenza. It’s often caused by a number of viruses, such as noroviruses or rotaviruses.

Common symptoms between gastroenteritis and flu include fever, headache, and body aches and pains.

In contrast to the flu, the symptoms of gastroenteritis are more focused around your gastrointestinal tract and may include watery diarrhea and abdominal cramps.

Pneumonia

Pneumonia is an infection of your lungs. While it can be a complication of the flu, there are also other causes. These include other viruses, bacteria, fungi, and certain chemical or environmental agents.

The common initial symptoms can be very similar to those of the flu and can include fever, chills, and headache.

Symptoms to look out for that can point specifically to pneumonia include cough with green or yellow mucus, shortness of breath, and sharp chest pain.

Bronchitis

Bronchitis is an inflammation of the bronchial tubes in your lungs. Like pneumonia, bronchitis can sometimes be caused by the flu virus. However, it can also be caused by other viruses or environmental factors such as cigarette smoke.

Overlapping symptoms between the two conditions include cough, fever, chills, and fatigue or malaise.

Similarly to pneumonia, symptoms to look out for that may indicate bronchitis include a cough with mucus, shortness of breath, and discomfort in your chest.

Food poisoning

You get food poisoning by consuming food that’s contaminated by pathogens, such as viruses, bacteria, or parasites.

Unlike the flu, symptoms are focused on your gastrointestinal tract and include nausea and vomiting, diarrhea, abdominal pain, and fever.

You may notice symptoms shortly after consuming contaminated food, although they may also take days or weeks to appear.

Lyme disease

Lyme disease is caused by a type of bacteria that’s spread through the bite of a tick. If left untreated, it can cause serious complications.

The early symptoms of Lyme disease can be very similar to those of the flu and can include fever, chills, body aches and pains, and fatigue.

Most people with Lyme disease also have a characteristic bull’s-eye rash at the site of the tick bite. However, the rash doesn’t occur in all people.

In some cases, Lyme disease has been mistaken for a summer case of the flu. If you’re experiencing flu-like symptoms and have a tick bite or have lived or traveled in an area where Lyme disease occurs, you should see your doctor.

You should see your doctor for your flu-like symptoms if you’re experiencing any of the following:

  • fever over 103°F
  • cough that includes yellow, green, or brown mucus
  • shortness of breath
  • pain in your chest, particularly when breathing in
  • lightheadedness, dizziness, or passing out
  • rash
  • persistent vomiting
  • flu-like symptoms that begin to improve but then return and are worse
  • flu-like symptoms in addition to a serious chronic condition, such as heart disease, lung disease, or diabetes

Although the influenza virus can circulate throughout the year, it’s most common during the winter months. If you’re experiencing flu-like symptoms during the summer months, it’s unlikely that you have the flu.

The best way to prevent getting sick over the summer months is to practice good health habits. This can include things like washing your hands frequently, covering your nose and mouth when you cough or sneeze, and avoiding people that are sick.

If you do have flu-like symptoms that become severe or are causing you concern, you should see your doctor to discuss your symptoms.