Seasonal flu is a nasty respiratory infection caused by the influenza virus. The flu generally rolls in with the cooler weather, circulating between September and May and peaking in January or February.You get the flu through exposure to the virus, which may happen when a person who is infected coughs or sneezes near you. Symptoms include cough, sore throat, chills, aches and occasionally fever. In some cases, the flu leads to complications such as pneumonia, ear infections, sinus infections and lung infections, and may even land you in the hospital. Thousands die (yes, die!) from the flu in the U.S. each year.MORE: 5 Ways to Fight the FluHow can you protect yourself from this nasty virus? According to the Centers for Disease Control and Prevention, the World Health Organization and the National Institutes of Health, the best way to prevent the flu is to get vaccinated. The organizations also recommend that virtually everyone—both healthy people and those at greater risk for the flu—should get a vaccination each year in order to get the best protection.According to Dr. Beth Ricanati, an internist on YouBeauty’s medical advisory board, mass immunization provides herd immunity, where people who are vaccinated not only protect themselves but also the unvaccinated people they come into contact with.“What I think is really important is that when you get the flu shot, it’s not just for yourself. You’re helping to protect those around you,” she says, noting that this is especially important if you spend time with the elderly, kids or people with compromised immune systems. “It’s great for us and it’s great for them. It is a public health issue as well as a personal health issue.”Dr. Ricanati recommends her patients to be vaccinated by the end of September or early October.But how exactly does the flu vaccine work? And how do you know which one to get? Building ImmunityThere are two basic types of flu vaccines: the flu shot and the flu nasal spray. Both are designed to prompt your immune system to recognize the flu virus so that when you are hit with the real thing, your body can stop it from infecting you.The flu shot accomplishes this because it contains inactivated flu viruses, which essentially means the viruses are dead. The body responds to these inactivated viruses by making immune cells called antibodies, which match up with targets called antigens that stud the surface of the virus. When the flu rips through your hometown or city in the winter, your new antibodies recognize its antigens, bind to them and neutralize the virus.The nasal spray works on the same concept, but instead of inactivated viruses, it employs live viruses that have been significantly weakened to the point where they can’t make you sick. This is called an attenuated virus, and like the dead virus in the flu shot, it primes your immune system to recognize the flu.For each flu season, the Food and Drug Administration decides which strains of flu are most likely to hit, and drug companies make their vaccines accordingly (seasonal flu viruses mutate from year to year. For more info, click here). For the 2013-2014 season, there are several versions of the flu vaccine available.The first set (given at once in one shot or spray) includes three variations on the standard flu shot, which all went through slightly different manufacturing processes and protect against three strains of flu selected by the FDA. There is also a high-dose shot and an intradermal shot that is injected into the skin rather than the muscle, both of which protect against the same three strains.For the first time, there are also two vaccines that protect against four strains of flu viruses. One is a standard flu shot and the other is the nasal spray vaccination.Which Is Right for You?That’s a lot of vaccine choices. Not all will be right for you, and some people may have medical conditions that mean they should avoid the vaccines altogether. Here is a quick guide on the shots, but as with any medical intervention, check with your doctor or pharmacist for a professional’s take.Out of the standard three-strain flu shots, there are three versions with different manufacturing processes. One, made by growing the flu virus in chicken eggs, is approved for ages 6 months and older. The other two are both egg-free. One is only approved for people 18 years and over, while the other is only intended for people between 18 and 49 years. The intradermal shot is only approved for people between 18 and 64 years.The high-dose three-strain flu shot is only for people 65 and up. It contains more virus than the other shots and was specially designed for this age group because they tend to have weaker immune systems. Whether the high-dose shot provides better protection for older people compared to the regular flu shot is not yet proven.Finally, the nose spray is only approved for healthy people between two and 49 years old. But, it comes with several cautions. The spray is not recommended for people with asthma or chronic issues of the lung, heart, kidney or liver, or with metabolism disorders. It also isn’t appropriate for pregnant women, children or teens on aspirin regimens, or anyone with a compromised immune system.Where Can You Get a Vaccine?Flu vaccines are generally available in pharmacies, health clinics and at doctors’ offices. To find a location near you, try entering your zip code into the HealthMap Vaccine Finder.