A perfect topic for flu shot season! Vaccines are injections (sometimes inhaled aerosols) designed to prime your immune system before a possible viral infection. Vaccines are made to give your immune system a leg-up on fighting viruses, like influenza, chicken pox, measles, etc. Vaccines contain either dead viruses or weakened, live viruses and sometimes extra material that your immune system recognizes as "foreign," basically the additive makes your immune system react more strongly when it encounters the dead or weakened virus. Your immune system encounters the virus in the vaccine, and the T-cells and B-cells which recognize it begin dividing like mad and making antibodies to it. Your body goes on Red Alert for that particular virus. Then, when you get infected with the real virus, your immune system is READY for it, and the virus can never really get going. Over time, the immune system response gets toned down a bit and your immunity fades. However, memory T-cells patrol for old viruses for many years to come - that's why one bout of chicken pox will keep you from getting it again for most of your life. Some vaccines require "booster" shots at certain intervals to keep your immunity up. Usually, weakened viruses make better vaccines than dead ones because your immune system reacts more strongly to the weakened live viruses. Some people can experience some infection-like symptoms from the weakened virus, but usually only if your immune system is depressed in some way. Your immune system can be weakened by not getting enough sleep, chronic stress, poor diet, some prescription medications, or another pre-existing illness. Your immune system truly is a remarkable, adaptable, multi-tasking, ever-ready virus killer.