here for a zoomable version).
Even though malaria was never a massive problem in the US as a whole it was certainly a problem in parts of the South as this map indicates. The darkest red color indicates a malaria mortality of over 1,400 per 10,000 - ie 1 in 7 people were dying of malaria.
In the US Civil War, where much of the action took place in these malarious states, large numbers of soldiers contracted Malaria and tens of thousands were killed.
The most shocking fact is that in 1870 no-one yet realized that mosquitoes were transmitting the disease.
Here's an excerpt from a Scientific American article entitled The Civil War and Malaria.
THE SCIENTIFIC AMERICAN’S ADVICE TO OUR SOLDIERS--MALARIA AND ITS REMEDIES
It is difficult for us to realize the fact, but we all know that any
soldier is in five times more danger of dying from malarious disease
than of being killed in battle.
WHERE MALARIA EXISTS
What malaria is nobody knows. It may consist of organisms, either animal
or vegetable, too minute for even the microscope to detect or it may be
some condition of the atmosphere in relation to electricity, or
temperature, or moisture; or it may be a gas evolved in the decay of
vegetable matter. The last is the most common hypothesis, but it is by
no means proved, and it has some stubborn facts against it. There is no
doubt, however, that malaria is some mysterious poison in the
atmosphere, and that it is confined strictly to certain localities.