There's this idea floating around that the "unnatural" immunity built up by receiving vaccinations is somehow not as good as the "natural" immunity gained by, I don't know, contracting smallpox. One of the arguments that is used to support this is that injecting a pathogen into the blood stream violates one of the first defences of your immune system: the skin. Whilst this is true, the skin is actually a pretty poor defence with all kinds of openings and weak points, and is only a tiny part of your bodys natural defences. Other evidence of the weakened immune system also include things such as short term swelling at the site and some mild flu-like symptoms. There are basically two branches of the immune system Innate and Adaptive. Innate is a very general type of immunity, consisting of the skin, white blood cells and causes symptoms like swelling and coughs to try and kick disease. It's a very "old" form of immunity, in that it developed early on in our evolution, and many pathogens have since adapted to the point that it can't fight them. We're not really concerned with the innate immune system when talking about vaccination as it doesn't "learn". Enter the Adaptive immune system. This is the one that can "remember" pathogens and provide lifelong immunity to a disease once it has been exposed. This is the system that we're mostly interested in. The point of vaccinations is that they expose the adaptive immune system to the pathogens that will get past the innate system, but in a less dangerous form. The immune system doesn't know this (it doesn't sit around reading scare articles on the internet) and learns how to attack it without having to chance it with a pathogen that could actually do some damage. This allows us to build up immunity without ever having to come down with full-blown diseases that could kill us.