Passengers from the sick plane were returning from Hajj to Mecca

A pair of airline health fears this week was linked to the Hajj pilgrimage to Mecca, officials said.

Eleven people were treated at a New York hospital after passengers and crew members were reported Wednesday during an Emirates flight departing from Dubai. The following day, 12 passengers on two flights to Philadelphia International Airport experienced flu-like symptoms.

"Overall, the common denominator is the case of influenza A in pilgrims returning from Hajj," said TIME TIME spokesman for the Centers for Disease and Control (CDC), Benjamin Haynes.

The hajj, an annual pilgrimage for practicing Muslims, this year attracted more than 2 million people to Mecca, Saudi Arabia. As with any event involving large crowds in confined spaces, outbreaks of communicable diseases such as influenza are common in hajj – this year is no exception.

Haynes says that three passengers from the New York incident and one from Philadelphia have tested positive for flu and / or other common respiratory diseases.

A Customs and Border Protection (CBP) representative told TIME that the medical journal in Philadelphia was done "with great care after Wednesday's illness of the crew and passengers of the Amirates Flight 203" .

Haynes says that health events involving air travel are relatively common and that the CDC has put in place "preparedness plans" for these incidents. Travelers should not cancel existing plans because of reported illnesses, says Haynes.

"Nobody wants to think about getting sick on a trip, but sometimes it happens. You may not be able to prevent all diseases or injuries, but you can plan ahead to be able to manage them, "Haynes said. "Make sure you're up to date with all your routine vaccinations, including the annual flu vaccine and specific vaccines at your destination before your trip, and make sure you practice healthy habits like hand washing.

People who are actively ill, however, should refrain from traveling if possible, Haynes says.

Also this week, Texas public health officials announced that passengers on four flights of Southwest Airlines may have been exposed to measles after a passenger was diagnosed with the virus in August.