I have asked this question before but I think it’s about time to ask it again; I’ve not, to date, heard a single good answer from any flavour of theist… if you’re a theist, let’s see if you can do any better than the usual nonsense…
First, the background:
Smallpox (the disease that inspired the first vaccine) no longer blights humanity; we have eliminated it (save some samples in controlled laboratories). Before we did eliminate it, though, it was responsible for the death of 1,000,000,000 people (that’s one billion)… it affected far more, causing pain, crippling, blindness and disfigurement. Of the 1,000,000,000 deaths, half were mere children. That’s half a billion children effectively tortured to death. That is far from an irrelevant consideration.
Now… from an atheistic and scientific worldview, there’s nothing really that needs explaining beyond the facts of the matter… Smallpox, like a great many viruses and bacteria, evolved. However… if you believe that a deity created the universe deliberately with full foreknowledge of what would happen… well… here’s the question you really ought to be able to answer:
What was the purpose of Smallpox? That is… why did the deity you claim exists create Smallpox, knowing that it would torture 500,000,000 children to death?
Sure, you can try to pretend that God’s perfect vision wouldn’t have included Smallpox if it weren’t for the fact of humans “sinning” (offending somebody’s imaginary fiend)… but then you have to recognize that what you are essentially saying is that God had innocent children tortured to death because of something their forebears did… does that sound just to you?