ATHENS (Reuters) – Eleven people have died and 107 infected by the mosquito-borne West Nile virus in Greece so far this summer, authorities said, the highest number of reported cases since 2012.

Outbreaks of the West Nile virus have been recorded in Greece since 2010, when they peaked at 262 cases including 35 deaths, according to Greece’s Centre for Disease Control and Prevention (KEELPNO).

In 2012, 161 people were infected and 18 of those died. Last year, the virus infected 48 people, of which six died. There were no reported cases in 2015 and 2016.

“There is no particular concern that we will have an uncontrolled spread of the West Nile virus, but the health ministry and KEELPNO are on alert,” Health Minister Andreas Xanthos told Greek radio.

The U.S. embassy in Athens issued a health alert urging citizens to take preventative measures, such as cleaning up mosquito breeding areas and keeping grass and bushes trimmed.

This year, cases were reported across Greece and all deaths were of people aged over 70.

Most people infected with West Nile virus experience no symptoms, meaning cases are likely underreported. About 20 percent develop fever and other symptoms, and less than 1 percent develop a serious illness, according to the U.S. Center for Disease Control and Prevention.

There is no specific treatment for the infection.

(Reporting by Karolina Tagaris; Editing by Raissa Kasolowsky)