West Nile virus killed 286 people in the United States last year, making it the deadliest year yet for the virus, the federal government reported on Monday.

Texas was especially hard hit by the virus, which is carried by mosquitoes and which only arrived in the United States in 1999, the Centers for Disease Control and Prevention reports.

“A total of 5,674 cases of West Nile virus disease in people, including 286 deaths, were reported to CDC from 48 states (excluding Alaska and Hawaii),” the CDC said in a statement.

West Nile virus is widespread in Africa, Asia and parts of Europe. It causes fever and aches and usually isn’t serious. But in some people it can spread to the brain or spinal cord, killing them or causing paralysis.

No one’s sure precisely how West Nile arrived in North America, but it was first reported in New York in 1999. It’s now been reported in all 48 contiguous states, as well as parts of Canada and Mexico and all the way to Argentina.

The CDC’s Dr. Lyle Petersen says it’s impossible to know what West Nile will do this summer. “It is very hard to predict,” he said in a telephone interview. “I can’t tell you what the weather is going to be like this summer, for example.” The virus is driven by weather; it’s worse during hot, wet summers in temperate climates.

“What last summer’s outbreak tells us is that West Nile is not going to go away,” Petersen said. “Most places in the United States are at risk of having outbreaks.”

The virus has a life cycle that takes it from mosquitoes to birds and back into mosquitoes that bite people. Its severity varies from year to year. In some years, only a few cases are reported, and in others, like 2012, it infects many people.

In 2011, CDC reported 712 cases of West Nile virus, and 43 deaths. The worst previous year was 2003, when 9,862 cases were reported with 264 deaths. Only severe cases are reported to CDC – health experts say most people who are infected don’t even know it. People over 50 and people with underlying illnesses are the most vulnerable.

“Last summer’s outbreak likely resulted from many factors, including higher-than-normal temperatures that influenced mosquito and bird abundance, the replication of the virus in its host mosquitoes, and interactions of birds and mosquitoes in hard-hit areas,” the CDC said in a statement.

“Because the factors that lead to West Nile virus disease outbreaks are complex, CDC cannot predict where and when they will occur.”

There’s no vaccine against West Nile virus for people, but there is one for horses. There’s also no specific antiviral drug that can help infected people – those who are seriously ill get what’s called supportive treatment in the hospital.

The CDC recommends that people avoid being bitten by mosquitoes, which can carry other diseases, also, from Eastern equine encephalitis to, in very rare cases, dengue fever. Standing water, even in extremely small containers, can breed mosquitoes. People should wear long sleeves and use insect repellent that contains DEET, picaridin, IR3535, and para-menthane-diol (PMD), CDC advises.

Petersen says scientists don’t quite understand why, but West Nile tends to cause more human outbreaks in temperate climates than in tropical zones. Female mosquitoes carry the infection over from one summer to another, he said. “Infected female mosquitoes find a warm place to live and they just hang out all winter,” he said. They often overwinter in sewers or basements, he said.

“These infected mosquitoes come out in the spring and then they look for a blood meal and they bite birds and infect birds. During spring and summer you get more and more infected mosquitoes and birds,” Petersen added.

“By the middle of summer there are so many infected mosquitoes that it starts to present a human infection risk.”

And because the virus can live in both mosquitoes and birds, it would be much more difficult to eradicate than malaria, which mostly infects people, Petersen said. The CDC helped eradicate malaria in the United States in 1951.