Government officials in Texas have ordered aerial spraying of insecticide in Dallas to kill mosquitos carrying the potentially deadly West Nile virus.

An outbreak of the disease this year has killed 26 people and sickened perhaps 700 more this year across a wide area of the U.S. Texas has been worst affected by what is seen as likely the worst outbreak in years.

William Reisen, a research entomologist from the University of California at Davis, says the current wave of infections is the largest number of West Nile cases officials have seen at this time of year since 2003.

West Nile symptoms mimic those of common diseases like flu, making it likely that official statistics understate the extent of infections.

About one in five people bitten by a mosquito carrying West Nile become ill. The disease becomes severe in less than one percent of those cases, and men and women over the age of 50 are the worst affected.

“[The disease] can produce long lasting memory loss, palsy, loss of limb use, memory loss and, if the progression is not stopped naturally, it can result in death.”

Reisen says research is under way to find a vaccine or other way to combat the illness, but so far “no good treatment” is available.

Some Dallas residents say they are worried that the spraying of insecticide could hurt their health. Reisen says research by one of his graduate students tracked hospital admissions before, during and after a similar program in California, and found no apparent impact.