It began with a mysterious outbreak of monkeypox, a disease that starts with a fever and headache, erupts into lesions covering the body, and then kills around one in ten people infected. As the outbreak spread, scientists discovered something strange. The strain had been genetically modified to be far more contagious than during previous flareups in central and west Africa. And it was resistant to the smallpox vaccine, one of the few defence measures against monkeypox.