The days get shorter, the air gets cooler and the leaves begin to change colors. Everyone knows that this signals the beginning of fall; what you may not know is that it also signals the beginning of flu season.

The flu is a contagious respiratory illness caused by the influenza virus. It is spread when viral particles from an infected person travel through the air from a cough, sneeze or even talking. Symptoms usually start one to four days after the virus enters the body; however, people who are infected can actually spread the virus a day before they feel sick.

Common symptoms of the flu include fever, chills, cough, sore throat, muscle aches and fatigue. The U.S. Centers for Disease Control and Prevention estimates that approximately 200,000 people a year are hospitalized for influenza in the United States and deaths from influenza can range from 3,000-49,000 per year. Some people are at higher risk of influenza hospitalization or death than others; these groups include children under five, adults over 65, pregnant women, residents of nursing homes and people with certain medical conditions, including asthma, chronic lung disease, heart disease, blood disorders and diabetes. Other individuals at risk for the flu include those with weakened immune systems, such as people with HIV and cancer and those individuals taking chronic steroids.

The best way to prevent flu infection is by getting a flu vaccine. Everyone over the age of six months should be vaccinated against the influenza virus. There are two main formulations of the vaccine: one given by an injection into the muscle. The other is given by a nasal spray.

Which vaccine is right for you? The nasal spray vaccine is approved for people between the ages of 2-49. Some people should not receive this form of the vaccine, including people with an allergy to the vaccine or eggs, pregnant women, those with weakened immune systems and children with asthma. For everyone else, the injectable flu vaccine is approved for ages six months and up. Those who have had an allergic reaction to the flu vaccine or to eggs should talk to a doctor before being vaccinated. Talk to your doctor or pharmacist about which vaccine is right for you and your family.

Can you get the flu from the flu vaccine? Neither flu vaccine can cause an influenza infection. However, since the flu vaccine takes 1-2 weeks before it is effective, being exposed to the influenza virus before or during this time may lead to infection. In addition, infections that are not influenza, such as a cold, can cause similar symptoms that are not prevented by the flu vaccine. There also may be strains of the influenza virus that are not covered in this year's vaccine.

What are the side effects? Some people experience mild side effects after the vaccine. For the injected vaccine, the most common side effects are soreness at the injection site and low grade fever. This represents the reaction mounted by the immune system. For the nasal vaccine spray, the most common reactions are a runny nose, sore throat and mild headache.

As we continue through this fall and flu season, make sure you're covering your mouth when you cough or sneeze, washing your hands frequently, and most importantly, that you get a flu vaccine. Since the influenza virus can spread before you even know you're sick, getting vaccinated doesn't just protect you--it also protects everybody around you.

For more information on influenza and the influenza vaccine, see:
The Defense Health Agency Immunization Healthcare Branch, http://www.vaccines.mil/flu
U.S. Centers for Disease Control and Prevention, http://www.cdc.gov/flu/protect/vaccine/index.htm