The virus landed in Boston on April 26, a Wednesday. It was brewing inside a young computer programmer who had flown in from India, brought over for his expertise by a financial services company headquartered in the city's tallest skyscraper.
He went to work on the 18th floor of the John Hancock Tower, and on May 5 the hallmark symptoms of measles began to appear: fever, cough, rash. Then, like a stone tossed into a lake, the disease rippled outward, with measles striking a half-dozen other workers at Investors Bank & Trust, five on the same floor.
By last week, four additional cases of the potentially lethal illness had been confirmed. Their link to the programmer is more tenuous, but city health authorities say they believe that all 11 cases in the state's first measles outbreak since 1999 can be traced to that single visitor.
The result: The state has distributed or ordered 23,000 doses of measles vaccine, at a cost of nearly $400,000. Hundreds of people at three workplaces have been ordered to stay home until they can prove they aren't susceptible or until they have passed the incubation period for the disease. And disease detectives have scoured medical records, examined office air-flow patterns, and conducted dozens of interviews in their quest to understand and stop the outbreak.
The arrival of measles in Boston, specialists said, illustrates the potential for dangerous germs to hitch a ride on a jetliner and travel from one corner of the world to another in a matter of hours.
It has happened before: Three years ago, SARS, or severe acute respiratory syndrome, hopped from Hong Kong to Toronto, spreading disease and death.
Doctors fear it will happen again: Many epidemiologists are much more concerned that avian influenza will be carried to the United States by humans than by birds.
``When we talk about globalization, it's not only globalization of goods and services; it's people and their history, including incubating diseases," said Dr. Gerald T. Keusch, a global health specialist at the Boston University School of Public Health.
``We can no longer think about putting up quarantines at the borders and expect that it's going to work for infectious disease any better than it works for the resourceful, determined people who come across as economic migrants," Keusch said. ``We can't put up a shield."
Even as Boston disease trackers attempt to stop the spread of measles, they are mining important clues about who is most vulnerable to a virus that once infected hundreds of thousands of children each year in the United States. They have found that eight of the 10 patients whose illnesses are believed to be linked to the programmer range in age from their early 30s to their late 40s. And that's telling.
Adults born before 1957 are believed to be universally immune to the respiratory illness because so many children in that era were exposed to the virus and developed natural and permanent protection. People under 30 grew up when vaccination was widespread and reliable.
When the measles vaccine was introduced in the 1960s, the pressure to vaccinate was so intense that infants would receive their shots at the age of 4 months, maybe 6 months . But doctors later realized that when babies are that young, they still have lots of disease-fighting cells from their mothers, and those antibodies sometimes neutralized the vaccine.
In other cases, to blunt the side effects of the early vaccine, doctors gave children a shot of another substance that, it turned out, weakened the effectiveness of the inoculation. As a result, the most vulnerable Americans are in their early 30s to late 40s.
``When you combine all of that, you get a small percentage of people who have remained not adequately vaccinated," said Dr. Adolf Karchmer, top infectious disease doctor at Beth Israel Deaconess Medical Center, who helped run measles vaccine campaigns for the federal government in the 1960s.
``Because measles has been so well controlled, you don't find out about those people, until you have what's happening in downtown Boston," he added .
When laboratory tests confirmed the first case in the current measles outbreak on May 12, doctors at the Boston Public Health Commission were alarmed. Knowing there was already one case in a business employing 1,500 workers meant there would almost certainly be more.
There was no indication that the computer programmer had been vaccinated against measles, disease investigators found. Studies by the World Health Organization show that in 2004, only 56 percent of infants in India were vaccinated against the condition, compared with 93 percent of US children.
It is not mandatory that workers coming into the United States on temporary work assignments prove they are vaccinated against diseases, federal officials said.
``The solution to vaccine-preventable diseases in the United States is very often improving global vaccine coverage," said Dr. Julie Gerberding, director of the US Centers for Disease Control and Prevention. ``There are still millions of children and adults around the world who are not protected against measles."
Two of the other local measles patients were also born abroad, one in Brazil and one in El Salvador, but those countries have childhood vaccination rates comparable to the United States.
Their ages have not been made public, and it's unclear whether they were ever vaccinated. Because of federal patient privacy rules, none of the patients in the measles outbreak has been named.
One day after the computer programmer's infection was confirmed, another worker at Investors Bank fell ill. Within nine days, five more became sick. The virus, propelled by a sneeze or a cough, can linger in the air for as long as two hours, still capable of infection.
But the outbreak didn't stop at the doors of Investors Bank. Although health officials cannot definitely link the later cases, ``there must be a connection," said Dr. Alfred DeMaria, chief medical officer of the state Department of Public Health. ``It would be an extraordinary circumstance to have another outbreak of measles simultaneously."
A man who works just down the street at the Christian Science church center developed measles. So did an electrical contractor who worked on the 56th floor of the Hancock Tower.
Dr. Anita Barry, director of communicable disease control in Boston, said officials might never find the precise link to the cases outside Investors Trust.
``With something like this, I'm not going to be surprised if we can't pin down the specific contacts," she said.
For example, one of the patients is an East Boston resident with no obvious link to any of the other patients. And then there's the man who clears tables at two restaurants, including Skipjack's, in the shadow of the Hancock.
``I would say 50 percent of our business during the lunch hour comes from the Hancock," said Sheldon Alman, general manager of Skipjack's, on Clarendon Street.
On May 31, one of Alman's employees approached him. The man, who has been whisking dishes to the kitchen for nearly two years, said he didn't feel well.
``I said, `Why don't you go home, and, if you don't feel well in the morning, call in sick and go see a doctor,' " Alman recalled.
A similar conversation had taken place a day earlier between the man and the manager of the other restaurant where he worked, Addis Red Sea, on Tremont Street. The worker had called in sick on Memorial Day.
All the patients in the measles outbreak are recovering and, so far, there's no indication the virus has spread to any diners at either restaurant.
Still, boards of health across the Boston area remain vigilant.
Stephen Smith can be reached at email@example.com.