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During the winter months, viruses and bacteria can spread more easily. Dry air helps germs spread. And we spend more time together indoors, breathing the same air.

Though common winter illnesses may make you miserable, knowing what’s causing your symptoms can help. That way, you can know how long your symptoms will last and when you should see a doctor.

Here are the eight most common illnesses during winter.

1. The Common Cold

More than 200 respiratory viruses cause the common cold. That’s according to the Centers for Disease Control and Prevention (CDC). The most frequent culprit is the rhinovirus, but the adenovirus and viruses in the coronavirus family can also cause the common cold.

The American Lung Association notes that adults get an average of two to four colds each year.

Common cold symptoms include:

• Coughing.
• A runny or stuffy nose.
• Sneezing.
• A sore throat.

Cold symptoms are usually at their worst in the first few days. You should start to feel better within a week to 10 days.

2. Influenza (The Flu)

The flu is a common winter illness that can come with serious symptoms for some people. About one in five Americans get the flu in a given year, with most cases occurring in the winter months. The flu hospitalizes hundreds of thousands of people in the U.S. each year.

On top of common cold symptoms, the flu causes fever, chills, headaches, body aches, fatigue, and sometimes nausea or vomiting. In severe cases, the flu can cause shortness of breath and dehydration. The flu can result in death.

People at risk of getting very sick from the flu include:

• People with chronic health problems (diabetes, heart or lung diseases).
• People who have a weak immune system.
• Pregnant people.
• Those older than 65.
• Very young kids.

If you’re at risk of getting very sick from the flu, call your doctor if you have flu symptoms. They can test you for the flu and offer medicine to lower your risk of getting severely ill from the flu.

If you’re otherwise healthy, you can treat flu symptoms at home unless they become severe. Rest, drink fluids, and take over-the-counter medicines like ibuprofen (Advil) or acetaminophen (Tylenol) for pain and fever. Symptoms should resolve in about a week, though cough and fatigue can last longer.

3. COVID

Though COVID-19 is less dangerous than when it first emerged, it can still lead to severe symptoms. Recent CDC data shows that COVID-19 hospitalizes twice as many people each year than the flu. COVID-19 can also lead to death.

Because the symptoms of COVID-19 and flu overlap, knowing whether you have COVID-19 or the flu without a test is impossible. COVID-19 can take about the same time to get better as the flu (a week or two).

Those at risk of severe disease from the flu are also at risk of severe disease from COVID-19. If you’re at risk of getting severely ill from COVID or flu and you’re showing symptoms of these viruses, see your doctor right away. After they figure out what you have, antiviral medicines can reduce your risk of severe disease, including pneumonia.

4. Respiratory Syncytial Virus

Respiratory syncytial virus (RSV) is a common virus that infects the nose and throat. In most people, RSV only causes common cold symptoms. It improves with home remedies like rest and fluids within a week or two.

But RSV can still pose a danger to young children, newborns, and preemies, as well as older adults, especially those over 65. Each year, RSV hospitalizes over 125,000 young children and over 60,000 adults over 65. RSV can result in death.

When severe, RSV can cause a high fever (above 103.0 F), shortness of breath, and wheezing. If you have these symptoms, get medical care right away.

5. Bronchitis

Bronchitis is when a respiratory virus infects the bronchial tubes. The virus that does this is either a common cold or the flu.

The bronchial tubes carry air to and from the lungs. More rarely, bronchitis can result from a bacterial infection.

About 5% of Americans get bronchitis each year.

Symptoms include:

• A cough that produces yellow or green mucus.
• Fatigue.
• A low-grade fever, below 101 F.
• A runny or stuffy nose.
• Shortness of breath.
• Wheezing.

In many cases, bronchitis doesn’t require medical care. Instead, you should rest, use a humidifier, and drink lots of fluids, like water, juice, and tea. Doctors don’t recommend over-the-counter cough-reducing medicine because coughing helps to clear the lungs of mucus.

Bronchitis can take up to 20 days to completely resolve, but the cough should improve in 10 days.

See a doctor if you have severe symptoms, trouble breathing, or your cough doesn’t improve in 10 days.

6. Pneumonia

Pneumonia is a viral or bacterial infection of the lungs in which mucus enters the air sacs. A fungal infection can also cause pneumonia, especially in people with weak immune systems.

Common illnesses during the winter, especially the flu and COVID-19, can infect the lungs and cause viral pneumonia. Respiratory viruses can also temporarily weaken the lung’s immune response to bacteria, leading to bacterial pneumonia. You might get symptoms of pneumonia days or weeks after you get symptoms of a cold, flu, or COVID-19.

Symptoms of pneumonia can include:

• Chest discomfort or pain.
• Cough.
• Fatigue.
• Fever and chills (a pneumonia fever is often high, above 103.0 F).
• Increased heart rate.
• Trouble breathing.

If you think you may have pneumonia, call your doctor. They can assess how severe your symptoms are. Some people with pneumonia must stay in the hospital for monitoring and treatment.

7. Sinusitis

Sinusitis is swelling in tissues that line the sinuses, which are air-filled spaces in the cheeks, forehead, and behind the nose. Common respiratory viruses can infect the sinuses, causing sinusitis. Bacteria can also get a sinus infection, though this is rarer than a viral cause.

A sinus infection from a virus usually gets better in days. Doctors diagnose bacterial sinusitis when symptoms persist beyond 10 days or get worse after a few days rather than better. Though bacterial sinusitis requires antibiotics, viral sinus infections get better on their own.

Symptoms of sinusitis include:

• Ear pressure or pain.
• Fatigue.
• The feeling of mucus dripping down the back of your throat
• Fever (this is more common with bacterial sinusitis).
• A headache or facial pressure.
• A runny or stuffy nose with yellow or green mucus.

8. Strep Throat

Strep throat is an infection of the tonsils and throat caused by group A streptococcus, a common bacteria.

Symptoms of strep throat include:

• Fever.
• Pain when swallowing.
• Red and swollen tonsils.
• Swollen lymph nodes under the jawline, near the ears.
• Tiny, red spots on the roof of the mouth.
• A very sore throat that comes on suddenly.
• White patches on the tonsils or pus at the back of the mouth on the tonsils.

If you think you have strep throat, your doctor can provide a rapid test. If your test is positive for strep, your doctor will prescribe antibiotics.