Production of antibodies are part of the body's defense mechanism. They help our immune systems fight off contagious disease and reinfection. This is the concept behind vaccinations. A vaccination, such as a varicella vaccine for chickenpox or the annual flu shot, is composed of inactive or non-infectious parts of a virus that will trigger an immune response in our bodies. In doing so, this causes us to develop antibodies that help defend us against the disease when we are exposed to the real virus. With some viruses, an initial infection causes our bodies to develop enough antibodies to give us natural, long-term immunity against a second bout of the same disease. This is why children who have had chickenpox (varicella-zoster virus), for example, don’t need to get the varicella vaccination for protection against a reinfection. Sometimes, however, our natural immune responses aren't enough to protect us against a disease. This is particularly true if we are exposed to high dose levels of a virus (a risk for healthcare workers), if the virus is particularly fast acting, or if our immune systems are compromised.