The U.S. is issuing an advisory urging Americans to avoid "unnecessary travel" to Mexico, where as many as 149 people have died of swine flu.
There are roughly 4,000 flights per week between the U.S. and Mexico. The Centers for Disease Control and Prevention is issuing the advisory "out of an abundance of caution," acting CDC chief Richard Besser said at a press conference today.
The European Union's health commissioner, meanwhile, appears to have irked U.S. officials by warning Europeans to avoid nonessential travel to both Mexico and the United States.
Besser called the U.S. travel warning "premature." He said that while 40 swine flu cases have been identified in the United States, there has been only one hospitalization for the disease.
There are 28 confirmed cases of swine flu in New York City, but Mayor Michael Bloomberg said the outbreak shouldn't dissuade tourists from visiting the city.
He emphasized that the cases are limited to a "single cluster" at St. Francis Preparatory School in Queens and that there is no need for people to wear masks.
"No one thinking of visiting New York City should think twice," Bloomberg said.
Airports in the United States and around the world, meanwhile, are on high alert as officials race to contain the swine flu outbreak.
At the U.S.-Mexico border, customs officials have been asking travelers whether they feel sick and, if so, have been referring them to health officials.
"What we're going to be doing at the borders, taking place starting today, is passive screening, asking people about fever and illness, looking for people who are ill, and handing out cards that let people know what's going on in Mexico, what's going on here, so people can take action to protect and prepare," Besser told "Good Morning America" today.
American airports are taking their own precautions. Officials at Los Angeles International Airport, for instance, said custodians have been sanitizing door knobs, handrails and faucets in restrooms.
More drastic measures are under way elsewhere in the world. In Japan, officials have been taking temperatures of those returning from Mexico. At the airport in Mexico City, people are wearing face masks, just as some are in the rest of the country.
U.S. airlines are also taking action. American Airlines said Sunday that anyone who bought a ticket to Mexico before April 25 for travel to or from Mexico won't incur a penalty for changing reservations. The waiver is in effect through May 6.
United Airlines announced a similar waiver, saying that passengers who bought tickets on or before April 26 for travel through April 30 may change their tickets without having to pay a penalty.
Fears about the impact of swine flu on tourism and travel sent airline stocks in Asia down sharply.
Stocks soared for pharmaceutical companies Roche Holding AG, of Switzerland, and Britain's GlaxoSmithKline as governments and corporations boosted their orders for their flu drugs, Tamiflu and Relenza.
With reports by ABC News' Dan Childs, the Associated Press and Reuters.