Washington ? West Nile virus may well complete its coast-to-coast spread this summer, infecting large numbers.

There’s no good way to predict, as the deadly virus is from a family that’s notoriously fickle. But during last year’s record-setting epidemic — more than 4,000 people became ill and 274 died — only a handful of states escaped human illness. Even some of those harbored infected mosquitoes and birds.

And no, the harsh winter in much of the country probably won’t lead to a reprieve. Many mosquitoes can survive the cold by hiding out in places such as sewers, ready to start spreading infection once it’s warm enough to re-emerge.

Tackling this virus “is unbelievably complex,” says Dr. Lyle Petersen of the Centers for Disease Control and Prevention, the nation’s chief West Nile specialist. The one sure discovery is that “where West Nile has been, it stays.”

He cautions that he has no crystal ball to predict whether West Nile, part of a family of mosquito-borne flaviviruses that can rapidly wax and wane, will prove as bad this year.

But Petersen says another large epidemic “would not be surprising,” with West Nile hitting each of the 48 contiguous states. Not counting Alaska and Hawaii, only nine states have escaped human illness so far.

West Nile can cause potentially fatal brain inflammation, either meningitis or encephalitis. It can strike at any age, but those most at risk are over 50.

There is no treatment, just supportive care. A vaccine for people will require years more research, although there is one for horses and some zoos are testing one for endangered birds.

Once infected, symptoms or not, you’re thought to be immune for life. So eventually, West Nile epidemics should become rare.