For the first time, traces of the West Nile virus have been found in mosquitoes in the UK, according to a report published this week by the UK Health Security Agency.

Here’s what you need to know about the virus and the disease it causes.

West Nile virus is a mosquito-borne virus first identified in Uganda in 1937. It belongs to the same viral family as dengue and yellow fever. The virus is most commonly transmitted by Culex mosquitoes, particularly the species Culex pipiens, which mainly feeds on birds.

Birds are the primary host for West Nile virus, and the virus spreads in a cycle from infected birds to mosquitoes and then back to birds. Occasionally, mosquitoes can transmit the virus to humans or other animals.

Most human infections – around 80% – cause no symptoms. When symptoms do occur, they are usually mild: fever, fatigue, headaches, body aches and sometimes nausea. But in rare cases, around one in 150 infections, the virus can cause severe illness, including encephalitis........

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