It has happened again, passenger jets landing back in the United States with sick passengers. Growing fears Friday for some passengers flying into this country from overseas.

Two more groups of travelers arrived in the United States with flu-like symptoms, raising fears that if you've been overseas recently, that flu bug may have been on the flight home with you.

Health officials and air travelers are on heightened alert. For a second day, passengers reporting flu-like symptoms, have been showing up in the U.S. from overseas.

And in Texas, Southwest Airlines says passengers on four of its flights may have been exposed to the highly contagious measles virus.

"Any time you have a closed environment like that where there's enormous amounts of people, these viruses can spread very easily," NBC Medical Correspondent Dr. John Torres said.

The latest scare, a dozen travelers saying they felt sick after landing in Philadelphia on two American Airline flights Thursday. One plane coming from Paris. The other from Munich.

But officials say all the sick passengers had been in Saudi Arabia attending the Muslim pilgrimage known as the Hajj in Mecca.  Both planes were detained with doctors evaluating everyone on board. The 12 passengers with symptoms were quarantined briefly. Health officials let them go after determining it was safe.

On Wednesday, an Emirates Airlines flight from Dubai, that stopped in Mecca, touched down at New York's JFK, reporting 19 passengers and crew felt ill.

"It felt like a flying infirmary," passenger Erin Sykes said. 

In Dallas, reporting a passenger was diagnosed with measles after taking four Southwest Flights last month,  the Dallas County Health Department is warning passengers to be on alert for symptoms.

"The best advice to keep yourself healthy is to make sure you have all your vaccinations including the flu vaccine before you get on that flight," Dr. John Torres.

Health officials mentioned being on the lookout for measles symptoms. Some of those symptoms include fever, a blotchy reddish-brown rash, a harsh dry cough where cold medicines don't help, muscle ache and fatigue.