8 people have acquired malaria in the US. They’re the first in 20 years.

In late May, Sarasota County, Florida, health officials confirmed they had identified a case of locally transmitted malaria. In mid-June, they confirmed the second. On June 26, after an additional two cases were confirmed, Florida health officials issued a statewide mosquito-borne illness advisory. So far in July, three more cases have been confirmed.

Meanwhile, Texas has reported one case of local malaria transmission: On June 23, its state health department announced the case was detected in Cameron County.

That’s a total of eight cases so far this summer. This is all highly unusual: Until now, the US hadn’t documented...