At least four times this week, in the span of just two days, health officials jumped into action to greet sick flight passengers.

An Emirates flight from Dubai landed at New York’s John F. Kennedy Airport on Wednesday morning with more than 100 passengers reporting symptoms including coughing, vomiting and fever. Eleven were taken to the hospital. That story got the most attention, but it didn’t stand alone.

An ASL Airlines France flight from Algeria to Perpignan in France was briefly quarantined Wednesday when a child aboard was suspected of having cholera. And two American Airlines international flights into Philadelphia on Thursday — one from Paris, the other from Munich — landed with a combined 12 passengers who were evaluated by health officials for influenza and respiratory problems.

When contagious disease symptoms present in the air, how do officials mobilize on the ground? What response mechanisms are in place? And how often are they activated?

There’s a complex international network in place at all times, combating the spread of infectious diseases. This includes the global reporting of diseases, how and where they spread and the integration of data.

It involves international, federal, state and local partners that, among other things, engage in surveillance of the travel experience, whether that be by land, air or sea, explained Dr. Marty Cetron, director of the US Centers for Disease Control and Prevention’s Division of Global Migration and Quarantine.

Cetron cited a “prescient report” released in 1992 by the Institute of Medicine (now the National Academy of Medicine) as the impetus for the network’s development. That report signaled that globalization, increases in international trade and travel, migration patterns, world conflicts and more would lead to emerging infectious diseases.

And so, since 1996, the CDC has been part of a mobilized effort to detect, protect and respond. Twenty US ports of entry — those with the highest volume of passenger arrivals and international travelers, including animals — are prioritized. But all ports of entry have points of contact.

What unfolded at JFK was part of a usually “hidden network of protection,” Cetron said. It’s one that, day to day, “is relatively silent to the US public, as it should be.”

It’s not entirely rare to see a medical response at an airport, he explained. New York City Acting Commissioner of Health Dr. Oxiris Barbot said her city sees five or six instances a year. But Cetron said the bulk of cases come up after passengers have moved on.

In most cases, 80% to 90% of the time, Cetron said, the CDC and its partners are working to re-create history after a flight. Someone ends up in a hospital with the measles, for example, and a health care provider concludes from the patient’s travel history that other flight passengers may have been at risk. That’s when the network kicks into gear, tracking down where the patient has been and who needs to be notified and screened.

It’s the same system that swung into action in 2014 after nurse Amber Vinson was diagnosed with Ebola she had contracted from caring for a patient in Dallas who later died from the disease. Vinson had flown from Dallas to Cleveland before her symptoms began and before she was diagnosed. Still the CDC and Frontier Airlines contacted all of the passengers on that flight about their health and well-being.

Sometimes, as was the case on the Emirates flight, Cetron said, a call came in to JFK 30 minutes before the plane landed. The airport, in turn, contacted the CDC. Enough people on the flight — many of whom were returning from the Hajj, the Muslim pilgrimage to Mecca — were exhibiting symptoms to cause alarm. And as they were coming from the Arabian Peninsula, the worst fear was the emergence of Middle East respiratory syndrome, or MERS.

An organized system was ready to respond. The jet was not brought to the gate, to prevent the possible spread of disease. A team of officials boarded to do an assessment. Specimens were collected. EMS, fire and rescue workers were there in case they were needed. All 549 people on board, passengers and crew, had a temperature check before most were free to go, said Caitlin Shockey, a CDC spokeswoman.

Eleven people were taken to the hospital; two tested positive for the flu, and one had a respiratory infection. All, with the exception of the one with flu and pneumonia, were released the next morning, Shockey said.

In the Philadelphia cases, there were also some sick passengers who’d come from the Hajj. Officials were not alerted to them, though, until a flight attendant raised the concern while the passengers were moving through customs. Cetron said gatherings that draw a couple million people are ripe for outbreaks.

The key is to not overreact or underreact, he said. “And when a system is well organized, we can respond like we did.”