Each autumn, everyone from your mom to your physician tells you to get a flu shot. That's necessary because every season the flu virus mutates slightly and your immune system can't quite identify it, which means that the virus has more time to infect you and make you sick. Of the three types of viruses, Influenza A can cause the most severe symptoms and can infect several different species, meaning that the virus can "jump" from animals like pigs or birds to humans. Flu vaccines effectively give your immune system a "wanted" poster based on researchers' best predictions for the mutation that year. The goal is that, when the virus arrives, antibodies will be able to bind to the virus and kill it before it can infect you.