Dallas Mayor Mike Rawlings declared Wednesday that the city is facing an emergency as the West Nile virus spreads, killing at least 14 people in Texas and 26 nationwide, according to the Centers for Disease Control and Prevention.

The Dallas declaration clears the way for aerial spraying to kill the infected mosquitos that carry the disease.

The United States is experiencing its biggest spike in West Nile virus since 2004, with 241 cases of the disease reported nationwide this year so far, including four deaths, health officials said last weekend, before the latest totals.

Of the 42 states that have reported infections in people, birds or mosquitoes, 80% of them have been in Texas, Mississippi and Oklahoma, the U.S. Centers for Disease Control and Prevention said in a statement. The CDC listed a breakdown of infections by state.

"It is not clear why we are seeing more activity than in recent years," said Marc Fischer, a CDC medical epidemiologist. "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 is transmitted through infected mosquitoes.

In the United States, most infections occur between June and September, and peak in August, according to the CDC.

Symptoms include fever, headache, body aches, joint pains, vomiting, diarrhea or rash.

"Less than 1% develop a serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues)," the CDC said.

Those at greater risk are people older than 50 and those with conditions such as cancer, diabetes and kidney disease, or with organ transplants.

There are no medications to treat West Nile virus or vaccines to prevent infection. People with milder illnesses typically recover on their own, but those more seriously affected may need hospital care.

Health experts say prevention measures include avoiding mosquito bites, using insect repellant and getting rid of insect breeding sites.

Symptoms of West Nile virus