Quarantined Aircraft and Sick Passengers. It’s Not as Scary as It Sounds.

A pair of flights from Europe landed at Philadelphia International Airport on Thursday and their passengers were sent for medical reviews. A day earlier, a plane from Dubai, United Arab Emirates, was quarantined briefly at Kennedy International Airport in New York after initial reports of as many as 100 sick travelers on board.

It sounds scary. Plane loads of passengers reporting flulike symptoms naturally raise concerns about a broader outbreak of disease. But the reports of quarantines and medical checks are actually evidence of a complex health system working exactly as intended, said the Centers for Disease and Control Prevention. And there’s a good explanation for why it all seemed to happen at once.

Basically, there’s no reason to worry. It might help, though, to know more about what happened.

Emirates Airline Flight 203

The flight from Dubai landed on Wednesday morning at Kennedy Airport and was quarantined. The pilot had alerted officials on the ground after dozens of passengers reported feeling ill, a law enforcement official told The New York Times on Wednesday.

The C.D.C. initially said about 100 of the passengers were thought to be sick, but after they were all screened, just 19 out of more than 500 travelers were deemed ill. Of these, 11 were taken to the hospital. They had the flu or a common cold, C.D.C. said.

American Airlines Flights 755 and 1317

Flight 755 came into Philadelphia International Airport from Paris and Flight 1317 flew in from Munich. Twelve passengers had flulike symptoms on the flights. The 250 passengers and the crew were held for medical review after leaving the flight. Apart from the 12 sick passengers, everyone was allowed to leave.

“Nobody was very ill, everybody was allowed to go on their way,” said Dr. Marty Cetron, the director of the global migration and quarantine division at the C.D.C.

Why were they all sick at the same time?

Many of the passengers had been on hajj, an annual pilgrimage that millions of Muslims make to Mecca, Saudi Arabia, to follow in the footsteps of the Prophet Muhammad. This year, hajj was in late August, and at least 1.7 million people are thought to have attended. Having traveled from all over the world, they were in close quarters for several days.

The spread of illness is to be expected. So is the fact that those who live in the United States took varied routes home (hence the illnesses reported on flights from the Middle East and Europe).

“Everywhere you have millions of people congregate, the disease transmission goes up,” said T.J. Doyle, the medical director at Stat MedEvac, which provides advice in cases of emergency like this.

It has happened before, but “the Saudi government takes great care in screening and treating,” said Dr. Cetron at C.D.C.

Is it serious?

No more serious than the regular outbreaks of flu that crop up in the fall and winter each year.

The quarantine of the plane at Kennedy Airport was highly scrutinized, but the system of responses worked exactly as they were expected to, Dr. Cetron said.

The center was called 30 minutes before the flight landed. The agency was then able to check global databases for how diseases were spreading globally. Local health services and laboratories were alerted to prepare for their arrival. The plane was stopped away from the terminal and people were checked and treated.

Will this be a more common occurrence?

Agencies like the C.D.C. are aware that people travel internationally. This kind of occurrence is relatively rare — it happens less than once a year, the C.D.C. said.

The agency spends much more time looking at people’s travel history to figure out where they were exposed because it can take some time for symptoms to be reported.

“The vast majority of our responses are reconstructing history and finding all the people across the U.S. and other countries, finding those most at risk, telling them how they can be tested and treated,” Dr. Cetron said.

Is there more risk of disease on flights?

It can vary depending on how infectious a disease is and whether it can be spread through the air or water droplets.

But the chance of picking up a disease on a plane increases with the amount of time you spend in the air, “just like if you were on a Greyhound bus for 10 hours,” Mr. Doyle said. “There’s nothing more inherently risky about being on a plane.”

Of course, traveling is always a little icky. A recent study of frequently touched surfaces at Helsinki Airport in Finland picked up traces of rhinovirus, the source of the common cold, and of the influenza A virus. They found traces on half the luggage trays, more than on any of the other surfaces they tested. None of these viruses were found on toilet surfaces at the airport, they said.

Airline staff and Customs and Border Protection officers are trained to spot and report sick travelers. If people are reported sick, the C.D.C. will vary its response depending on how infectious the disease is.

What about when people are on hajj?

The C.D.C. recommends that anyone going gets vaccinated if they can and makes sure their family is protected because vaccination does not prevent people from carrying pathogens.

And this is just as important for a common illness, like influenza, as it is for any other disease. “Influenza kills thousands of people each year,” Mr. Doyle of Stat MedEvac said. “More people die of the flu than die of ebola.”