US Department of Agriculture (USDA) researchers studying cultures of African swine fever virus (ASF) under a microscope. ASF virus causes African swine fever, a haemorrhagic fever in pigs that has a high mortality rate and is threatening the food security of billions of people worldwide. USDA researchers have identified several genes that are necessary for the virus to cause disease. Virus particles with these genes removed are candidates for use in vaccines that could protect livestock against the disease. USDA researchers have engineered four such viruses, which are undergoing further investigation. Photographed at Plum Island Animal Disease Center, New York, USA, in 2019.