I thought it was anything that could raise your body temp up for an extended time....
People used to spreading some of these, what are now terrifying, diseases on purpose just to get it out of the way.
Multiple families with kids would get together with someone who has the disease and they all would get it....get it over with...and move on.....it was called having a "party"
Now!!! People die! We have nothing to fear but fear itself!