Midafternoon on a recent Friday, the New Jersey chapter of the American Academy of Pediatrics e-mailed its membership that it would hold a webinar on measles. By the time registration closed at 6 p.m. that Sunday, 219 doctors had signed up for the next morning's presentation - more watched in groups - and another session had to be scheduled for later in the week.

Fifteen years after measles was officially eliminated from the United States, most young doctors have never seen an actual case. So, as confirmed measles nationally surpasses anything witnessed in decades, pediatricians and other primary-care doctors are striving to learn all they can - examining photos of the classic rash online, questioning older colleagues in offices and hospital hallways, calling public health authorities for more information.

"I only have read about it in textbooks. That's very different from seeing something on a daily basis," said Jennifer Coren, a pediatrician for 15 years. Her small practice in Hatboro held an emergency staff meeting two weeks ago to review symptoms and policies. "I can tell you pretty quickly and with pretty good accuracy whether you have flu or strep," she said. "I can't do that for measles."

Even with the recent Disneyland outbreak that unvaccinated visitors carried back to their home states, now up to 125 cases, measles is an uncommon disease. Just one case each has been reported in Pennsylvania, New Jersey, and Delaware since Jan. 1 (none linked to the California outbreak).

Most cases do not produce serious illness. What concerns doctors are measles' potentially major complications and, in particular, its extreme infectiousness. A baby under one year, when the first of two vaccine doses is scheduled, is almost certain to be infected simply by being in the same room as someone with the illness.

To prevent that, pediatrician Sharon Sowinski-Mueller's office in Warrington has a new policy.

"Anyone who is calling in with fever and conjunctivitis" - pink eye, in this case in both eyes - "we are asking the parents to drive them over and call us when they are there so we can go out and look at them," she said. If the doctor sees a constellation of certain other symptoms, including a cough, runny nose, and rash - and hears about recent travel and incomplete immunization - the child will be sent home to be isolated and the county health department notified of a possible measles case.

"I'm confident that I could make a diagnosis, but I have that small bit of worry in the back of my head that it might be later in the process compared with someone who had experience seeing it before," said Sowinski-Mueller, who completed her residency in 2003.

Experts say that thinking about measles is the best way to recognize it. No one was doing that in 2001 when a woman in her mid-20s arrived in the emergency room of the Hospital of the University of Pennsylvania with fever and a rash, then developed pneumonia. The ER doctors, wondering about other possibilities, requested a consult with Todd Barton, a young infectious diseases physician.

Measles wasn't on his mind, either, until he did some reading. History was a clue: The patient had traveled from overseas, and recently been at large, public gatherings in Philadelphia. And, while she had been immunized with two doses as a child, the second was not given in the recommended time frame.

"The diagnosis is not hard to make once you suspect the disease," Barton said.

The challenge facing physicians now is different from in the past.

"Twenty years ago, you wouldn't go to the doctor until a rash appeared," said Meg Fisher, medical director of Unterberg Children's Hospital at Monmouth Medical Center in Long Branch, who led the webinar for New Jersey pediatricians. Now, "people don't like children with fever," and bring them in.

Even after several days of fever, cough, and red eyes, measles is indistinguishable from adenovirus or flu. A blood test at that point will often be negative.

The first clinical evidence is the appearance inside the cheek of little dots, like grains of sand, called Koplik's spots.

"There is nothing else besides measles that gives you those spots," said Fisher, who is triple-qualified on measles. She is a pediatric infectious diseases subspecialist, a "gray hair" who treated multiple cases during the 1989-91 outbreak (which was more than 50 times bigger than what has been seen in the last year), and remembers having it herself at age 7.

The classic rash that starts on the face and moves down and across the body doesn't appear until a couple of days later, as the Koplik's spots fade away. By that time, the child has already been infectious for four days.

Since 99 percent of the earlier symptoms turn out to be something else, the only real clues are known exposure, like a recent visit to Disneyland, and incomplete vaccination. A combination of common symptoms, exposure, and lack of immunity can still lead to overreporting as a precaution.

Daniel Vigil, a family doctor and associate professor at the University of California, Los Angeles, School of Medicine, did that three times recently. "Those folks were quarantined," he said. "All were negative tests. We erred on the side of caution."

Forty-four percent of pediatricians in the United States have entered the workforce since measles was technically eliminated in 2000; far more began practicing in the years since measles was common. Vigil began in the mid-1990s and works in the state with the biggest outbreak, yet has not seen a case.

Caroline Johnson, disease-control director at the Philadelphia Department of Public Health, said she is contacted several times a month by physicians who say, " 'I don't really think this is measles, but I want to run it by you,' and we are very happy to get those calls."

Perhaps five times a year, the history and symptoms call for isolating the child and sending a blood sample to the state lab. It is usually negative.

Johnson doesn't expect an outbreak in the city because the measles immunization rate for young children is 95.9 percent, well above the estimated 92 to 94 percent needed to achieve "herd" immunity.

Rates for New Jersey and the rest of Pennsylvania are slightly lower but in a safe range, according to the latest National Immunization Survey. Still, spotting measles quickly is critical.

"An infant who is unvaccinated is protected in general in the city," Johnson said. "But an infant exposed to a case in a waiting room has no immunity at all."

Measles Advice for Doctors

In her webinar for physicians, pediatric infectious diseases subspecialist Meg Fisher discusses a key concern about measles - it is so contagious that a baby too young to be vaccinated is 90 percent likely to be infected simply by being in a room with someone who has the disease.

Some points she makes about infection control:

Triage is essential.

When possible, make the diagnosis outside of your office: car or hospital.

If the child is in your office, put in separate room.

Mask on the child if possible.

The area is considered contaminated for two hours after the patient leaves.

Airflow in the office: Air may be recirculated.