The West Nile virus has sickened at least 31 people in 13 states so far this summer, according to the U.S. Centers for Disease Control and Prevention. Three people have died from the mosquito-borne infection.
There is no vaccine or treatment for the virus, which can cause deadly brain swelling. The disease is primarily transmitted to humans by mosquitoes, but can also spread through blood transfusions and organ transplants.
"It is important to note that these methods of transmission represent a very small proportion of cases," the CDC says on its website, noting that mothers can also transmit the virus to their babies during pregnancy or birth and through breast milk.
Blood is routinely screened for West Nile, according to the American Red Cross, which has detected roughly 1,600 infected donors since 2003. But at least nine people have been infected by transfusions from donors with very low levels of the virus in the past decade, according to the organization's website.
Nine of the 31 people who have tested positive for the virus this summer are thought to have donated infected blood, according to the CDC.
Four out of five people with West Nile develop no symptoms at all. But one in five of those infected reports a fever and other symptoms such as head and body aches, vomiting, diarrhea and a rash,
according to the CDC.
Although treatment is limited to symptom relief, most people recover completely from the infection within weeks or months. Less than 1 percent of patients develop neurological symptoms, which can be permanent and even deadly.
"People over 60 years of age are at the greatest risk for severe disease," the CDC says on its website. "People with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants, are also at greater risk for serious illness."
Symptoms can take two weeks or longer to appear, according to the CDC.
Mosquitoes pick up West Nile by feeding on infected birds. Humans cannot get the virus by handling dead or live birds, but are urged to check with their state health department on whether to report dead birds or dispose of them safely.
West Nile cannot be transmitted from person-to-person.
Mississippi and South Dakota have reported at least seven cases each, according to the CDC. Infections have also occurred in Alabama, Arizona, California, Colorado, Georgia, Iowa, Minnesota, Nebraska, Nevada, Tennessee, Texas and Washington.
Click here for more information from the CDC.