How do secondary infections happen? “It could be the immune system gets tired and another infection is able to come in,” says David Weitzman, MD, an urgent care physician in South Carolina and a board member of the American Academy of Urgent Care Medicine. “Or you’re exposed to a second type of virus or bacteria and get sick again.” RELATED: The Flu: Self-Treat, See a Doctor, or Go to the ER?

Adults 65 and Older People in this age group account for 50 to 70 percent of seasonal flu-related hospitalizations and between 70 and 85 percent of seasonal flu-related deaths. (2)Young Children As many as 26,000 children 5 and under in the United States wind up in the hospital due to the flu each year. (3)Pregnant People Normal changes to the immune system, heart, and lungs during pregnancy can raise the risk of severe illness from the flu. The flu may also be harmful to a developing baby. The CDC advises people who are pregnant or planning to become pregnant to get the annual flu shot. (4)Adults With Preexisting Conditions These include chronic illnesses such as asthma, chronic obstructive pulmonary disease, heart disease, diabetes, and HIV or AIDS. (5)American Indians and Alaska Natives Research has shown these groups experience a disproportionate number of flu-related hospitalizations compared with the general U.S. population. (6)

RELATED: 8 Home Remedies to Stop a Bad Cough

Bronchitis

Bronchitis — inflammation of the airways that carry air to and from the lungs — is commonly caused by cold and flu viruses, according to the National Heart, Lung, and Blood Institute. Bacteria and inhaled irritants can also cause bronchitis. The most common symptom is coughing laced with mucus. Others include shortness of breath and wheezing. (7)

Dehydration

Colds and the flu can affect the appetite, decreasing the amount of food and fluids a person takes in. This can lead to dehydration. Other factors can include symptoms such as diarrhea, vomiting, and fever, according to the Mayo Clinic (8). Signs of dehydration in infants or young children include listlessness, no wet diapers for three hours, a dry mouth and tongue, and no tears when crying. Adults may develop extreme thirst, urinate less, produce darker-colored urine, or feel dizzy, confused, or fatigued. (9) Severe dehydration is a medical emergency and may require treatment with intravenous fluids.

Ear Infection

“Earaches are common with colds, because of inflammation of the eardrum,” Dr. Rosenfeld says. Children are especially susceptible to middle-ear infections because their Eustachian tubes (which connect the ear and back of the nose) are shorter, floppier, and more horizontal than those of adults, which makes it easier for bacteria in the back of the nose to reach the ear.

Myocarditis

Myocarditis, inflammation of the middle layer of the wall of the heart (the myocardium), can be caused by cold or flu viruses, as well as the novel coronavirus that causes COVID-19, according to the Mayo Clinic. It may begin with mild symptoms, such as fatigue or shortness of breath, but it can lead to arrhythmia (rapid or abnormal heart rhythms), heart attack, or stroke. (10)

Myositis

Viral infections like influenza can cause inflammation that damages muscle fibers, a condition called infectious myositis, according to Harvard Health. The main symptom is weakness, but muscle aches and tenderness aren’t uncommon. Myositis usually subsides on its own in a few days or weeks. (11)

Pericarditis

Pericarditis, inflammation of the sac of tissue surrounding the heart (pericardium), is commonly caused by viral infections including colds, the flu, and COVID-19, according to the American Heart Association. The symptoms of pericarditis are sharp chest pain resembling a heart attack as well as fever, weakness, and coughing. Pericarditis usually gets better with little or no treatment, but severe cases require hospitalization and surgery. (12)

Pneumonia

As with COVID-19, cold and influenza viruses can lead to pneumonia, a common and potentially life-threatening lung infection. These viruses can lead to viral pneumonia or bacterial pneumonia, with the latter usually being more serious, according to the American Lung Association. (13) Symptoms of pneumonia include cough, fever, shortness of breath, rapid and shallow breathing, and nausea and vomiting (especially in small children). If untreated, pneumonia can become severe, with signs such as difficulty breathing, rapid heart rate, and a bluish tint to the lips and nail beds, according to the American Lung Association. (14)

Sinus Infection

Acute sinusitis, which is inflammation of the sinuses that causes a buildup of mucus, is most often caused by a cold, according to Harvard Health. The most common symptoms are congestion, headache and facial pain, and discolored mucus. Others include loss of smell or taste, cough, bad breath, fever, toothache, and the sensation of fullness in the ears. (15) RELATED: No Flu Shot and Now You Have the Flu: What to Do Learn More About How to Tell the Difference Between a Cold and the Flu For instance, taking too much acetaminophen (Tylenol) can damage the liver. According to the Cleveland Clinic, adults should not take more than 4 grams a day. For people with liver conditions, the limit is 2 grams per day. (17) Aspirin has been associated with a risk of Reye’s (also called Reye) syndrome in children and teenagers who have a viral infection such as the flu. Reye’s syndrome is a rare but serious condition that causes swelling of the brain and liver damage, according to the Mayo Clinic. (18) Prescription antiviral drugs for treating the flu may have side effects. The most common side effects of oseltamivir (Tamiflu), for instance, are nausea and vomiting. (19)

Trouble breathing or shortness of breathPersistent pain or pressure in the chest or abdomenPersistent dizziness, confusion, or inability to rouse from sleepSeizuresNot urinatingSevere muscle painSevere weakness or unsteadinessFever or cough that improves but then returns or worsensWorsening of chronic medical conditions with fever and a worse cough

Emergency warning signs for children with the flu include:

Rapid breathing or trouble breathingBluish lips or faceRibs pulling in with each breathChest painSevere muscle pain (child refuses to walk)Dehydration (no urine for eight hours, dry mouth, no tears when crying)Not alert or interacting when awakeSeizuresFever above 104 degrees; in babies under 12 weeks, any feverFever or cough that improves but then returns or worsensWorsening of chronic medical conditions

Many of these symptoms resemble those seen with COVID-19, so the CDC recommends people who develop them be tested for COVID-19, particularly if they’re at risk for serious illness from the virus. As with a cold and the flu, people 65 and older and anyone with a chronic medical condition —including asthma (moderate to severe), diabetes, cancer, heart disease, stroke, being overweight or obese, and/or a weakened immune system — is at increased risk for serious illness from COVID-19. RELATED: How Do You Handle a Flu Emergency? Getting diagnosed right away is also key, particularly in light of the risks associated with COVID-19. If you have any of the following symptoms, you should get tested to determine whether you have a cold, the flu, or COVID-19:

FeverChillsCoughShortness of breathFatigueMuscle achesHeadacheLoss of taste or smellSore throatCongestionRunny noseNausea or vomitingDiarrhea