LOS ANGELES (AP) — California health officials say 28 people have died from West Nile virus so far this year.

As of Friday, 517 people in 30 counties have tested positive for the virus this year. West Nile is transmitted to humans and animals by the bite of an infected mosquito. Flu-like symptoms can occur.

Many of those who died this year were senior citizens, who health officials said are at a higher risk of getting sick and developing complications.

Last year, a record 31 people died from West Nile.