An electron micrograph of the West Nile virus, commonly found in Africa, western Asia and the Middle East, but also now present in the USA, Australia and parts of Europe. The virus can infect humans, birds, horses and some other mammals. Only around one in five infected humans will develop a fever, and of these, only one in 150 cases will be severe. However, severe infections can lead to encephalitis, meningitis and polio-like symptoms.
Cynthia Goldsmith/Centers for Disease Control and Prevention