A relentlessly rainy spring and President Donald Trump’s trade war with China aren’t the only forces haunting the Midwest’s corn and soybean farmers. A deadly, highly contagious disease called African swine fever—thankfully, harmless to humans—is sweeping through China’s hog farms, literally killing demand for feed. African swine fever has already wiped out at least 20 percent of the nation’s hog herd this year, according to the Dutch agricultural lender Rabobank.