Flight crews and passengers traveling overseas have been put on alert by the Centers for Disease Control and Prevention (CDC).

Twenty-two U.S. airports are posting warnings about MERS – or Middle East respiratory syndrome coronavirus – and what they can do to avoid infection.

The CDC says the risk of disease for airplane passengers is extremely low, but believes international travelers should educate themselves about the signs and symptoms of MERS and the regions most affected by the spread of the disease.

“The bottom line is being very cautious, particularly if you’re going to the Arabian Peninsula and have any reason to work in a health care center,” Dr. Marty Cetron, director of global migration and quarantine at the CDC, told FoxNews.com. “That’s really the epicenter of the epidemic.”

A second case has been identified in The Netherlands. Both cases involve family members who traveled together to Saudi Arabia.

“It is also known that both patients have underlying conditions that make them probably more susceptible to infection with this virus,” said the health ministry in a statement.

According to the World Health Organization, 571 MERS cases have been reported worldwide – 171 of them fatal.

MERS symptoms mimic flu symptoms — fever and signs of lower respiratory illness, such as coughing and shortness of breath. The CDC says people who develop symptoms within 14 days of traveling from countries in and around the Arabian Peninsula should seek medical attention.

“MERS-CoV, at the moment, is not readily transmissible, except in very distinctive circumstances. The most important of which is in the healthcare environment, where the health care providers have very close, prolonged contact with the patients,” said Dr. William Schaffner, professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center.

Two cases have been diagnosed in the U.S. and the CDC has contacted approximately 500 airline passengers who may have come in contact with the patients.

“We’re still learning about the transmission characteristics of the coronavirus, but we don’t have all that information yet, which is why we do these types of investigations. The operating theory and hypothesis is through [human-to-human] contact, but how people are getting it from animals, we don’t understand that yet,” said Cetron.

A spokesperson for U.S. Customs and Border Protection (CBP) said the agency has been working with the CDC to identify travelers with MERS.

“When a traveler or alien is identified with a possible communicable disease or identified from information that is received from the CDC, CBP personnel will take the appropriate safety measures by donning personal protective equipment (PPE), to include gloves and surgical masks, which are readily available for use in the course of their duties,” said Jaime Ruiz, a spokesperson for CBP, in an email.

Like any other transmissible disease, there are precautions that passengers can take to ensure they stay healthy while flying.

“Wash your hands regularly, avoid sick people and avoid environments that are high risk,” Cetron said. “Know the signs and symptoms, and if you return from the Arabian Peninsula and you get sick, be sure to call your doctor; say that ‘I was just in Saudi Arabia, where can I be tested for MERS?’”

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