Two hundred forty-one cases of West Nile have been reported to authorities so far this year, according to the CDC, exceeding any pre-August figure since 2004.
"It is not clear why we are seeing more activity than in recent years," said Marc Fischer, a disease specialist with the CDC.
"Regardless of the reasons for the increase, people should be aware of the West Nile virus activity in their area and take action to protect themselves and their family."
The virus can cause serious neurological problems or death in a small number of cases. It has no treatment or preventive vaccines.
Nearly 80 percent of U.S. cases were reported in Texas, Mississippi or Oklahoma, according to the CDC.
Read more on the bulletin here.