"You're welcome. Did I explain vaccination or just mention it...? I think I just mentioned it in passing. People throw off diseases that are caused by germs because there's a thing called an immune system in our bodies that figure out what things are germs - as opposed to just random stuff that isn't trying to hurt us - and kicks them out before they do damage. You might know of some diseases that you can only get once in your life? Chicken pox, maybe? Those are staying pretty much the same, so once you 'learn' what it looks like it never bothers you again. Other diseases change their details so you can get slightly different ones and be just as sick dozens of times. Colds are like that. And it turns out that if you take some germs that you've killed, or weakened - or even germs from a similar but less dangerous disease, in one case I can think of - and then you put them where your immune system can get a good look at it and easily beat it up, it'll know what to look for next time. Also, fun fact, allergies are what happens when your immune system gets the wrong idea about what things are germs and freaks out over nothing."