SAN JOSE, CALIF. - 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 conclusion of a decadelong analysis by University of California, Santa Cruz, biologist Marm Kilpatrick, who explored the ecology of an infectious disease that killed five Californians last 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 the most familiar birds, its numbers have surged along with the popularity of lawns at homes, parks and schoolyards. Kilpatrick dubs them "super-spreaders."

Origin in Africa

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 probably smuggled itself into America within an infected mosquito on a transatlantic airplane. The original New York strain seemed to match a strain found in Israel, suggesting Mideast origins. But now it's different.

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

Four years to West Coast

Then, within a mere four years, the new strain landed on the West Coast.

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.

Although the virus can infect a wide range of animals, the robin seems to play the major role in transmission. Crows also can be disease carriers. Millions of birds have died after 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 new conclusion are reported in last week's issue of the journal Science. Kilpatrick and colleagues caught mosquitoes and then studied the blood in their stomachs -- identifying, through DNA, that it tended to belong to robins.

They also identified which type of mosquitoes are the key transmitters of West Nile. The species that usually bites us isn't the culprit. This helps explain why infection has stayed relatively limited.