I'm not sure about the TB thing, but as far as vaccinations go, the injection contains a weakened or genetically similar strain of whatever they are inoculating against. This allows your body to develop antibodies on it's own, so when the real infection is met, your body will know what to do with it. Similar to getting chicken pox. You get them once, your body fights off the attack and remembers how. In the future, should you be exposed to the pox again, your body will already know how to defend itself and rarely will you have another infection.