DES MOINES, Iowa (AP) — Officials have announced the year’s first confirmed death from West Nile virus in Iowa. The Iowa Public Health Department said in a news release Friday that an adult woman from southwest Iowa died earlier this month.

The state medical director, Dr. Patricia Quinlisk, says the West Nile virus threat remains until the state’s first hard frost. The virus is transmitted to humans through mosquitoes, which acquire the virus by feeding on infected birds.

Experts say most people who are infected have no symptoms or experience only mild, flu-like symptoms. The most vulnerable people are those who are at least 50 or have weakened immune systems.