Four people have died from the disease. While 42 states have reported finding the virus in either mosquitoes, birds or people, 80 percent of the human cases have been in three southern states, Texas, Mississippi and Oklahoma.
The disease typically peaks in mid-August, the CDC said. The last time so many cases had been reported through July was 2004.
"It is not clear why we are seeing more activity than in recent years," said Dr. Marc Fischer, an epidemiologist with CDC's Arboviral Diseases Branch. "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."
Fischer said the best way to prevent infection is to avoid mosquito bites. Protective measures include using insect repellent, wearing long-sleeved shirts and long pants at dawn and dusk, when mosquitoes are most active, and using air-conditioning to keep cool instead of open windows.