The U.S. is on track to have the most cases of measles this year in more than a decade: 118 through May 20.
Most of the virus is imported, from southern Asia, Africa and Europe, where vaccination has been hampered both by fears of vaccine side effects and immigration from areas with low vaccination rates.
Measles is highly contagious -- up to 90 percent of people exposed to someone with measles can get sick. The virus spreads readily through the air, and droplets can linger in a room for two hours after the infected person leaves.
The disease has long been a child killer, taking a terrible toll in less-developed countries. An estimated 2.6 million a year died until widespread vaccination campaigns sharply reduced fatalities attributed to the virus, but the toll is still estimated to be well over 100,000 children a year.
The measles, mumps and rubella vaccine is routinely recommended for all American children at 12 to 15 months, with a second dose given at 4 to 6 years of age. One dose can be given to a baby as young as 6 months who is going to travel overseas. Among adults with no evidence of immunity, one dose is usually recommended unless they work in health care or are regularly around international travelers.
Coverage worldwide is far from the 95 percent, two-dose rate needed to block disease transmission. Almost 60 percent of countries have 90 percent coverage with at least one dose, international health officials say.
The U.S., which declared measles eliminated as a homegrown disease in 2000, claims vaccine coverage of 90 to 95 percent. But it only takes one unvaccinated person traveling overseas or one foreign visitor to spend time in a place where a lot of unvaccinated people pass through to make trouble.
Unvaccinated people accounted for 89 percent (105) of the 118 cases diagnosed thus far, according to the Centers for Disease Control and Prevention.
Incomplete surveys done for the CDC in recent years suggest that several hundred thousand schoolchildren have medical or philosophical exemptions for getting at least one childhood vaccine otherwise mandated to attend school, with pockets of unprotected kids as great as 30 percent in some communities.
Other surveys suggest up to a quarter of toddlers under age 2 are incompletely vaccinated, due either to parental concern about "vaccine loads" or to poor access to or management of pediatric care.
The U.S. residents infected so far this year include 45 youths, ranging from 12 months to 19 years. Of those, 39 were not vaccinated, including 24 whose parents claimed a religious or personal exemption and 8 who missed vaccination opportunities.
Health officials particularly worry about measles infection in very young children or among adults who have other health problems and have never been vaccinated. Both groups can have more severe symptoms and even die from the infection - usually the disease is fatal to one or two children out of 1,000 who are infected.
Officials also are concerned that far too many health care workers may not be sufficiently protected against measles.
Consider the outbreak in Tucson in 2008, documented in a recent report in The Journal of Infectious Diseases. One unvaccinated and infected traveler was admitted through a hospital emergency room and then to a room, but measles was not confirmed for eight days.
The traveler set off a cascade of nearly 400 suspected measles cases, but just 14 were confirmed. All 14 of those patients were unvaccinated.
But the impact was even greater among Tucson hospitals and their staff members; 30 percent were found to have no documentation they'd been immunized for measles or had natural immunity. Thousands were sent home either because they lacked immunity or may have been sick. Nearly 4,500 health workers at seven hospitals had to be vaccinated.
Public health officials who reviewed the study say it's unlikely many U.S. hospitals would be any better prepared to deal with an in-house measles outbreak or to determine which staff members might be at risk.