Hot weather are unusual people died, including an epidemic of new types or epidemic ever disappear from this world, and then it comes back, we have seen the disease carriers and new release adds a lot.
The unusually hot weather until someone died. The outbreak of a new type. Or diseases that had disappeared from the world, then came back to us to see the new one. And the growth of disease vectors.
Hot weather, unusual until someone died. The new type of plague The disease had disappeared from this world and come back to see new And the carrier that add more.