Fifteen years ago an unwelcome viral visitor entered the U.S., and we've been paying for it ever since.

The U.S recorded its first case of West Nile virus back in 1999. Since then, the disease has spread across the lower 48 states and cost the country around $800 million, scientists reported this week in the American Journal of Tropical Medicine and Hygiene.

The hefty bill works out to about $57 million a year. And the tab doesn't include money spent to control mosquitoes that spread the virus.

The high price tag comes mostly from the treatment of people who develop serious neurological problems when the virus infects the nerves. In these cases, West Nile can cause seizures, coma, paralysis of limbs (something called acute flaccid paralysis), swelling in the brain or spinal cord (encephalitis and meningitis) and even death.

These illnesses from West Nile virus are rare, occurring in less than 1 percent of infections. But when they happen, they can be nasty — and expensive.

Treatment may involve multiple trips to the hospital and long-term physical therapy. Symptoms can linger for months, even years.

To figure out how much it costs to treat these severe forms of West Nile virus, epidemiologist J. Erin Staples and her team at the Centers for Disease Control and Prevention tracked the initial medical cost of 80 Coloradans hospitalized for the illness back in 2003.

Five years later, they followed up with about half of those patients to estimate long-term medical expenses incurred from doctors' visits, physical therapy sessions and hospital stays. They also asked the people about how much work they missed from being sick.

The team then used an algorithm to extrapolate from those 80 cases to the 37,000 recorded across the country since 1999. The result? Treating West Nile virus over the past 14 years has cost the U.S. somewhere between $678 million to $1.01 billion.

But Staples and her team didn't stop there. They also figured out which types of neurological problems — such as meningitis or acute paralysis — were most expensive.

People with meningitis had shorter hospital stays than those with paralysis or encephalitis, the team found. But younger people tended to get meningitis. So they missed more work, and the meningitis cases were associated with a higher economic cost in lost productivity. On the other hand, people with encephalitis tended to be older and retired. So lost productivity was lower for these cases.

But the biggest economic cost of treating West Nile virus, by far, was decades of work lost when people died of the disease.

Since 1999, West Nile virus has caused the deaths of at least 1,549 people in the U.S. Altogether, Staples and her team estimate that these premature deaths cost the U.S. about $450 million since 1999 — or nearly half of West Nile's total tab.

So how does the virus' price tag stack up against other diseases? "It's hard to compare," Staples tells Shots. "The economic impact on a population really depends on the number of cases that you're seeing."

Take for instance, Texas in 2012, she says. The state recorded a whopping 1,868 cases that year. "So I'd wouldn't be surprised if West Nile virus wasn't one of the more costly diseases for Texas that year."

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