I have a scar from a smallpox jab, when I was 7. We were due to go on a family holiday to France and there was an out break there at the time, hence the whole family had the injection.
I had a very bad reaction and was ill for 2 weeks. Doc said it meant I would very likely have succumbed to smallpox, if I had caught it.
We don't realise how lucky we are these days. All those diseases that used to be common such as TB, smallpox, polio, diphtheria etc.,