A previously unrecognized severe febrile illness in China has been traced to a novel bunyavirus, possibly transmitted by ticks. Characterized by severe fever and thrombocytopenia, the illness had an initial case fatality rate of 30%. Since June 2009, investigators have documented the presence of the virus in 171 patients from six regions in central and northeastern China, and the infection proved fatal in 12% of cases. The virus invaded multiple cells, but primarily thrombocytes and leukocytes. Multi-organ failure developed rapidly, as reflected by elevated liver enzymes, creatine kinase, and lactate dehydrogenase. A causal relationship between the virus and the illness has yet to be established. However, epidemiologic, clinical, and laboratory data strongly implicate the virus in the febrile illness, the researchers reported online in the New England Journal of Medicine.