The number of reported cases of West Nile virus in the U.S. through the first three weeks of August, according to the Centers for Disease Control. That puts the country on course for the worst outbreak ever of the tropical disease, which first made its way from Africa in 1999. So far 75% of all cases have been reported in five states—Texas, Mississippi, Louisiana, South Dakota, and Oklahoma—with Texas responsible for about half the cases overall, but there’s been a case reported in nearly every state in the U.S. This year’s outbreak of West Nile disease—which is spread by mosquito bite—has also been particularly severe, with 41 deaths so far.

(MORE: Why West Nile Virus Is a Self-Inflicted Wound)

As I wrote earlier this week on Going Green, global warming—specifically increasing temperatures and erratic rainfall—have made conditions perfect for the invasion of West Nile, but climate isn’t the only factor behind the spread of this virus and other tropical diseases in the U.S.:

But things aren’t as simple as just “hotter temperatures equals more disease.” That’s because there’s another factor at work: us — or, more specifically, our policymakers. The severity of tropical diseases is also a matter of whether or not governments are capable — and willing — to defend their populations against infections. In a 2010 paper in Nature, researchers noted that even though global temperatures increased throughout the 20th century, the range of malaria actually contracted, as countries developed economically and put in place measures to control the disease. Malaria was once endemic in southern U.S. states like Florida and Georgia until after World War II, when it was eradicated thanks to the work of the CDC, which was created in 1946 with the stated goal of controlling the disease. Today tropical infections like malaria and dengue are as much diseases of poverty as they are of climate, which is why nearly 200,000 people die from malaria each year in the desperately poor Republic of Congo, while the disease has been eliminated in the tropical but rich island nation of Singapore. It isn’t being hot alone that kills — it’s being poor.

(MORE: Access to Contraception Helps Save Lives—and the Planet)

Bryan Walsh is a senior editor at TIME. Find him on Twitter at @bryanrwalsh. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME