context: Even if you're usually healthy, the flu can knock you off your feet for days -- even weeks. And it doesn't always happen, but there's a chance that it could lead to more serious health problems, or "complications," like sinusitis (sinus infections), bronchitis, or pneumonia. But if you know what the symptoms are and how to take precautions, you can avoid these problems and stay healthy. It's a very contagious disease caused by the influenza virus. People tend to catch it most often in the fall and winter. It comes on fast and strong, spreading through your upper respiratory tract and sometimes invading your lungs. You may have: Fever (usually high Headache Tiredness (can be extreme) Cough Sore throat Runny or stuffy nose Body aches Diarrhea and vomiting (more common among children than adults) They include viral or bacterial pneumonia, dehydration, and ear infections and sinus infections, especially in children. The flu can worsen long-term medical conditions, like congestive heart failure, asthma, or diabetes. You might also have muscle inflammation (myositis), problems with your central nervous system, and heart problems such as heart attacks, inflammation of the organ (myocarditis), and inflammation of the sac around it (pericarditis). Adults over 65 Children ages 6 months to 4 years Nursing home residents Adults and children with heart or lung disease People with compromised immune systems (including people with HIV/ AIDS) Pregnant women It can happen when the flu virus enters your lung or when you get a bacterial infection during the course of the illness. Pneumonia can make you quite ill and may send you to the hospital. It can cause chills, fever, chest pains, and sweating. You might have a cough with green or bloody mucus. You could notice a faster pulse, and your lips or nails might have a bluish tint because of a lack of oxygen. Other symptoms include shortness of breath and sharp pains in your chest when you take a deep breath. Seniors may only notice a pain in the belly. When you get a bacterial infection with the flu, your symptoms may get better at first. Then they get worse with high fevers, more coughing, and a greenish tinge to what you're coughing up. Call your doctor if you have a cough that won't stop, a bad fever, or if you get shortness of breath or chest pains. The doctor can do tests to find out if you have pneumonia. Antibiotics can treat bacterial pneumonia, but these meds can't treat viral pneumonia. It can hang around for about 2 weeks, or even longer in young children, elderly adults, and those who have weakened immune systems or ongoing illness like COPD or asthma. Even healthy people may feel tired or weak for a month or more after their lungs clear up. There are 2 types: pneumococcal polysaccharide vaccine ( PPSV23) for adults and the pneumococcal conjugate vaccine ( PCV13) for children. The adult vaccine protects against 23 types of bacteria that commonly cause pneumonia. Doctors suggest that healthy seniors over 65 get both vaccines. The timing and sequence in which you get them will vary depending on what vaccines you've already had. Some experts say adults younger than 55 should get both vaccines to boost their immune system. The pneumonia vaccine isn't recommended for pregnant women. But it can help people at increased risk for infections, like those with: Heart disease Liver disease Lung disease Kidney failure Diabetes Some cancers Sickle cell anemia HIV/ AIDS Asthma (or smokers) ages 19 through 64 Children under age 2 should get four doses of the PCV13 vaccine. Tots between 2 and 4 who didn't get the pneumonia vaccine series should get a single vaccine. Children 6 to 18 with health problems should get a single dose of PCV13 whether they had shots already or not. Call him if you have a high fever and a hard time breathing. Other serious symptoms include: Fever with shaking chills Coughing with blood-tinged mucus from the lungs Trouble breathing Rapid breathing Shortness of breath Chest pains Wheezing Many can be managed. But some, depending on how weak your immune system is, can't be prevented. If you do get the flu, call your doctor within 48 hours after your symptoms show up. Ask about a flu antiviral drug. If you get them early enough, they can help ease your symptoms and help you get well sooner.