So there is something odd going on with measles. I haven't dug into the data, but from a couple of data points I've seen, it seems that measles mortality was declining for a long time even while measles cases were not. That is, there was a decline in the case-fatality rate, without a decline in cases. The disease was still around but getting less deadly. Then the vaccine actually reduced the number of cases.

So what was reducing the case-fatality rate? I don't know, but it might have been nutrition. There's evidence at least that Vitamin A makes measles less severe/deadly.