All "pox" diseases are caused by viruses. Unlike bacteria, which can be treated with antibiotics, there are no magic bullets to deal with viruses, available from the doctor. Even antivirals will not remove the virus though they would help you to overcome the infection. Your immune system uses white blood cells to destroy viruses and it does so very effectively. Some white blood cells eat up the virus particles and other produce special proteins called antibodies which attach to the virus particle and destroy it. The antibodies are special because they are 'tailor made' to deal with the particular virus eg. chickenpox. They will only work against it and no other. More importantly, you never lose the ability to produce a particular antibody once you have made it once. This is because other white blood cells carry a 'memory' of all the antibodies you have ever made. Vaccines work by giving you a version of a virus which has been inactivated but which will still cause an immune response (ie. antibody production). You generate antibodies against the vaccine, and then forever carry the memory of how to make them again, in case the active version of that virus gets into you, sometime in the future. This happened to you as a baby when you received your chickenpox vaccination. You should therefore be unable to ever get chickenpox and even if you did, you would still be far better protected than you would have been if you hadn't been vaccinated. Anyone who is unlucky enough to get chickenpox despite having been vaccinated could do so if their immune system is either faulty or compromised in some way. The only other way that you could get it, would be by the virus mutating so that the smallpox antibodies would no longer be able to recognise it as their target - but then it would be a new virus. Some viruses like colds and flu are constantly mutating - that's why they always come back. Yeah so. That's it.
