Officials in Dallas, Texas, have stepped up efforts to combat West Nile virus after the deaths of 10 people. The West Nile Virus has struck all over the continental United States in recent weeks, but Dallas has been the hardest hit.

Officials there are fumigating on the ground and in the air, targeting specific mosquitoes which spread the disease.

West Nile virus was first identified in 1937 in Uganda in eastern Africa. It was first discovered in the United States in the summer of 1999 in New York. Since then, the virus has spread throughout the United States.

The West Nile virus is a type of virus known as a flavivirus. Researchers believe West Nile virus is spread when a mosquito bites an infected bird and then bites a person.

Mosquitos carry the highest amounts of virus in the early fall, which is why the rate of the disease increases in late August to early September. The risk of disease decreases as the weather becomes colder and mosquitos die off.