I didn’t read the entire article, but perhaps the locals to the region had contracted malaria several times during their lives while living there and so were immune to its effects, while the Brits were not.
Although many of the Americans were from up north and came with Washington, so I suppose they got it as well.
Malaria is caused by a parasite in your blood stream.
Back then when you had it, it would regularly symptomatically flair up once contracted it. You don’t get immune, like mumps or measles. It was a very bad thing,
BTW - World War II: Many troops had to suffer casualties by inflicted malaria even in World War II. Gen. Douglas MacArthur’s predicament in May 1943 is very clear: “This will be a long war if for every division I have facing the enemy I must count on a second division in hospital with malaria and a third division convalescing from this debilitating disease!” It appears that the general was not at all worried about defeating the Japanese, but was greatly concerned about the failure to defeat the Anopheles mosquito! 60,000 U.S. troops died in Africa and the South Pacific from malaria. U.S. Forces could succeed only after organising a successful attack on malaria.