Like any new immigrant, the deadly West Nile virus became American almost immediately after landing on our shores — altering itself to fit in and then infecting a popular backyard bird to secure a firm foothold in its new home.

That is the startling conclusion of a decade-long analysis by UC Santa Cruz biologist Marm Kilpatrick, who explored the ecology of an infectious disease that has killed five Californians this summer and sickened another 197, up from 82 last year.

“Just like other invasive species, the virus starts adapting to its new environment,” Kilpatrick said.

The West Nile virus has done so well because it took advantage of a species that thrives around people: the American Robin. One of our most familiar birds, its numbers have surged along with the popularity of lawns in homes, parks and schoolyards. Kilpatrick dubs them “super-spreaders.”

It is a cautionary tale. As the earth grows warmer, pathogens may find new hosts, aided by jet travel, global trade and immigration.

The West Nile virus was discovered in Uganda in 1937. In some parts of Africa, more than 80 percent of adults have been infected with the disease that can cause fever, diarrhea, body aches and vomiting. For reasons not well understood, many never become ill.

Shortly after arriving in New York in 1999, the virus started changing — evolving to become a new and distinct strain, the research shows. It likely smuggled itself into America within an infected mosquito on a trans-Atlantic airplane. The original New York strain seemed to match a strain in Israel, suggesting Middle East origins. But now it’s different.

This new strain is transmitted more efficiently by local mosquitoes than the original strain.

Then, within merely four years, it landed on the West Coast. By 2005, the new strain had completely displaced the African strain.

More than 1.8 million people have since become infected in North America, with about 360,000 sicknesses and 1,308 deaths, according to Kilpatrick. The outbreak also has meant the costly creation of a national blood donor screening system, as well as vaccine and drug development.

And although the virus can infect a wide range of animals, the robin seems to play the most major role in transmission. Crows, another habitué of our backyard, also can be disease carriers. Millions of birds have died from West Nile infection.

Understanding the spread of West Nile virus could help experts design strategies to prevent the spread of future foreign pathogens, Kilpatrick said. Rift Valley Fever and Japanese encephalitis already loom on the horizon.

The average American tends to be sanguine about the outbreak of exotic diseases in faraway places, perhaps because we trust oceans to separate us from much of the world.

But it worries the nation’s leading health officials. Migrating birds sweep across North America, carrying disease with them. So medical and agricultural inspectors work around the clock to spot new threats.

“West Nile virus has provided a test of our ability to respond to such a spread,” said Sam Scheiner, director of the Evolution and Ecology of Infectious Diseases program at the National Science Foundation. The UCSC research provides knowledge “that can be applied to the next threat.”

The new conclusions, published in last week’s issue of the journal Science, summarize research from many different sources.

Kilpatrick and colleagues caught mosquitoes and then studied the blood found in their stomachs — identifying, through DNA sequencing, that it tended to belong to robins.

They also identified which type of mosquitoes are the key transmitters of infection. Here’s the good news: The species that usually bites us isn’t the culprit. This helps explain why infection has stayed relatively limited.

“Instead, it’s the species that feed mostly on birds but occasionally feed on people,” he said.

If you’ve been bitten by a mosquito, how do you know whether it was the type that carries West Nile? Unfortunately, because mosquitoes look so much alike — particularly after they’ve been squashed — the average person would have a tough time making an identification, he said.

In California, most of the human cases tend to be in the Central Valley and Los Angeles and Riverside counties — areas that are warmer and wetter, due to agriculture, and have more mosquitoes, Kilpatrick said. Perhaps, too, residents are outside more at dusk when mosquitoes bite.

“The foggy and cooler summers in the coastal areas,” such as Santa Cruz, San Mateo and San Francisco, “are much less conducive” to disease, Kilpatrick said. No human cases have been reported this year in those three counties.

The season typically ends in late October. It takes a long cold snap to wipe them out.

Nationally, the worst years were 2002 and 2003, which leads to this question: Will West Nile virus follow the boom-and-bust pattern seen in some other plant and animal invasions?

“It’s not known yet how much of the year-to-year variation can be explained by climatic conditions,” Kilpatrick said, “or other factors, such as acquired immunity in birds or human.”

Contact Lisa M. Krieger at 408-920-5565.