context: Influenza, or flu, is a virus that targets the respiratory system. Here are answers to the 10 most common questions about the flu. The flu and the common cold are both respiratory illnesses, but they are caused by different viruses. Influenza or "the flu" develops when a flu virus infects the respiratory system, including your nose, throat, bronchial tubes, and possibly the lungs. A cold virus usually infects only the upper respiratory tract: your nose and throat. The flu usually causes more severe illness than the common cold. Flu can bring on fever, body aches, and exhaustion, symptoms that are rarely caused by simple colds. Primary symptoms of flu are fever, fatigue, body aches, chills, headache, sore throat, and cough. The cough is a bronchial tube irritation and is usually not productive -- you're not coughing up gunk. The flu is usually at its worst for three to four days. The cough may linger longer. Recovery may take seven to 10 days. You may have lingering fatigue for several weeks. There's one catch with flu viruses. About 24 to 72 hours after you're infected, you become contagious. Yet you may not have symptoms, so you don't know you're sick. You feel completely healthy and go about your daily affairs -- spreading the virus wherever you go. Stay at home while you have the flu, and for at least 24 hours after your fever is gone without the use of fever-reducing medicine. Once your fever is gone for a day, you're no longer contagious and can return to work or school. Also, you will recover more quickly if you get some rest. There's no single "best" treatment for flu, but there are many ways you can ease symptoms. Prescription flu drugs can shorten the time you feel sick if taken when symptoms first appear. They work best when taken within 48 hours of symptoms, but they may also prevent severe disease if taken more than 48 hours after the first symptoms. Over-the-counter cold and flu drugs can offer some relief from fever, aches, stuffy nose, and cough. They don't "cure" the flu, but may help keep you more comfortable. What can help? Decongestants can help you breathe by shrinking swollen mucous membranes in the nose. But talk to your doctor first if you have high blood pressure or heart disease. Saline nasal sprays can also help open breathing passages. Cough preparations, along with water and fruit juices, can help soothe a cough. If you use a multi-symptom cold medicine, choose the drug that matches your symptoms. And don't take two cold medicines with the same ingredients. Don't use over-the-counter cough and cold drugs in children under 4. If your child is between 4 and 6, ask your doctor before giving any medicine. It's safe to use these drugs to help relieve symptoms in kids 6 and older. Never give medicines with aspirin to chilodrenunder the age of 19 due to the risk of Reye's syndrome. It's very important to drink a lot of fluids to keep the body hydrated. This helps loosen mucus. Limit drinks like coffee, tea, and colas with caffeine. They rob your system of fluids. As for eating, follow your appetite. If you're not really hungry, try eating simple foods like white rice or broth. The prescription drugs baloxavir marboxil ( Xofluza), oseltamivir ( Tamiflu), zanamivir ( Relenza), and peramivir ( Rapivab) were developed to cut short a bout with flu. They help shorten recovery time by one or two days. The drugs work best when taken within 48 hours of the first symptoms. However, clinical studies show the drugs still offer benefits when treatment starts more than 48 hours after symptoms begin. Oseltamivir ( Tamiflu) and zanamivir ( Relenza) can also be taken to help prevent the flu if you're been exposed to the virus. Antibiotics will not help treat the flu or a cold. Antibiotics kill bacteria, but they do not kill any viruses, including viruses that cause the flu or colds. However, the flu can weaken the immune system and open the door for bacterial infections. If your flu starts to get better and then gets worse, you may have a bacterial infection. See a doctor right away. Antibiotic treatment may be necessary. If your symptoms are severe or aren't getting better after a week or if you are over 65 or have fever for more than 3 days, see a doctor. Also call a doctor right away if you have any chronic medical conditions and are exposed to the flu or develop any of the symptoms. Moreover, if an infant or young child has a fever or develops symptoms of the flu, get medical help. These symptoms are signs that flu may have developed into something serious like pneumonia. See a doctor right away if you have any of these symptoms: Difficulty breathing Persistent fever Vomiting or inability to keep fluids down Painful swallowing Persistent coughing Persistent congestion Persistent headache Because the flu virus can infect the lungs and it can cause a serious infection like pneumonia. And that's what worries people. If the flu develops into pneumonia, it may require hospitalization and can even lead to death. People with weak immune systems -- the elderly, pregnant women, infants, and people with chronic health problems -- are at highest risk of flu complications such as pneumonia. Others at high risk for complications include Native American and Alaskan Natives. The flu shot is made from dead viruses and cannot "give" you the flu. However, the vaccine can trigger an immune response from your body, so you may have a few mild symptoms, like achy muscles or a low-grade fever. The nasal flu vaccine, Flu Mist, is made with a weakened live flu virus. It also cannot give you the flu, but is more likely to cause symptoms such as achy muscles or a low fever. It's recommended as an option only for non-pregnant, healthy people between the ages of 2 and 49. Flu and cold viruses are transmitted the same way -- through microscopic droplets from an infected person's respiratory system. That person sneezes or coughs, and droplets are sprayed onto any nearby surface -- or person. If they cough or sneeze into their hands (without a tissue), their hands then carry droplets to surfaces they touch. You touch that surface and pick up the virus. If you rub your eyes or nose, you've just infected yourself. To help protect yourself and prevent spread of cold and flu viruses: Wash hands frequently. Use an alcohol-based gel if you don't have access to water. Cough and sneeze into a tissue or inside the bend of your elbow if you don't have a tissue. Wash your hands afterward. When you cough, turn your head away from others. Don't touch your eyes, nose, or mouth. This prevents germs from entering your body. Wash and disinfect any shared surfaces (like phones and keyboards) frequently. Viruses can live on surfaces for up to 24 hours. Stay away from crowds during cold and flu season. Get a flu vaccine every year. Vaccines don't give you 100% protection from the flu, but they're the best way to help prevent it. Eat healthy foods to nourish your immune system, such as dark green, red, and yellow vegetables and fruits. Get regular exercise. People who exercise may still catch a virus, but they often have less severe symptoms and may recover more quickly. Also, regular exercise -- aerobics and walking -- may boost the immune system. People who exercise regularly tend to get fewer colds. They may also recover more quickly than people who do not exercise regularly. Check with your doctor before beginning a new exercise program. No, allergies don't affect susceptibility to the flu. But people with asthma are more likely to have complications, such as pneumonia, when they do get the flu. Also at risk of complications are infants under age 6 months, pregnant women, people with suppressed immune systems, people with diabetes, people with lung disease, people with neurologic disease, people with heart disease, and the elderly.