The death toll from the West Nile virus in the United States rose this week to 87 people total in 2012, up from a total of 66 deaths reported the previous week.
According to the U.S. Centers for Disease Control and Prevention, the number of cases rose from 1,590 to 1,993 in the past week. The figure represents the largest number of cases of the virus since it appeared in the U.S. in 1999, BBC reports.
Texas has experienced at least 495 cases of the most serious neuroinvasive form of the disease and has reported at least 40 deaths.
Cases of the disease peaked in 2002 and 2003 when the most severe cases of West Nile hit close to 3,000.
Approximately one in five people infected with the West Nile virus gets sick and one in 150 of them will develop more severe symptoms. The symptoms include disorientation, neck stiffness, coma and paralysis, according to BBC.
While West Nile virus cases have been reported in 47 states, approximately 75 percent of the cases are from five states, Oklahoma, South Dakota, Louisiana, Mississippi and Texas.
Health officials believe that a mild winter transitioning into an early spring increased the breeding of mosquitoes that bite infected birds and transmit the virus to humans, BBC reports.