West Nile virus is surging across the U.S. this summer, with infections running about 40% higher than normal, according to new reports.The virus is most commonly spread to people through the bite of an infected mosquito, although the disease cycle also includes infected birds. When mosquitoes feed on infected birds, they can pick up the virus and transmit it to humans.There have been roughly 770 human cases in the U.S. as of early September, with nearly 500 classified as severe, according to the latest data from the Centers for Disease Control and Prevention, which tracks cases reported by state health departments.KANSAS TRACKS 5 SERIOUS WEST NILE VIRUS CASES AS MOSQUITO SEASON PEAKS ACROSS THE STATEThat's about 40% higher than the average number of cases reported by this point in previous years (around 550 total and 350 that qualify as severe), based on CDC data.Most West Nile cases are reported in August and September.The official report emphasized that this data is still preliminary. Many mild infections go unreported, and theres often a lag before all severe cases are confirmed.MOSQUITO-BORNE VIRUS SPREADING THROUGH CHINA CAUSES HIGH FEVER, JOINT PAINThough a spike is expected during peak season, experts say the intensity of the outbreak and the percentage of mosquitoes testing positive for the virus is unusual.For most people, West Nile produces mild symptoms such as fever, aches and rashes or none at all, according to the CDC.In rare serious cases, the virus invades the nervous system, causing meningitis, encephalitis (swelling of the brain), paralysis or death.Its the spike in these latter symptoms that is causing concern. People over 50 or those with compromised immune systems are at highest risk, experts caution.West Nile virus was first identified in Africa in 1937, with the first European outbreak occurring in 1996. It reached the U.S. in 1999, when an outbreak in New York sent 62 people to the hospital over two months, data shows.CLICK HERE TO SIGN UP FOR OUR HEALTH NEWSLETTERAvoiding mosquito bites is the first line of defense. Health officials recommend wearing loose, long-sleeved clothing; applying insect repellent (especially at dawn and dusk when mosquitoes are most active); and ensuring that window screens are secure.MORE IN HEALTH NEWSAround homes, eliminating standing water (like that found in buckets, tires and gutters) can help keep mosquitoes from breeding.