73 cases if swine flu confirmed, global fear mongering begins.

- Seventy-three cases of swine flu have been confirmed worldwide, the World Health Organization said Monday.

Forty of those cases are in the United States, 26 in Mexico, six in Canada and one in Spain, a WHO representative said.

Hundreds more cases are suspected, especially in Mexico, where as many as 103 deaths in Mexico are thought to have been caused by the virus, the country's health minister said. More than 1,700 cases have been reported but not confirmed in the country.

Federal officials confirmed 20 new U.S. cases on Monday.

A federal official said they were at the same school in New York in which eight U.S. cases were confirmed earlier. More than 100 students at the school were out with flu-like symptoms last week.

President Obama said Monday that the swine flu outbreak is a "cause for concern and requires a heightened state of alert," but is not a "cause for alarm."

He added that the federal government is closely monitoring emerging cases and had declared a public health emergency as a "precautionary tool to ensure that we have the resources we need at our disposal to respond quickly and effectively."

Meanwhile, the European Union's health commissioner Monday called on people to avoid traveling to both the United States and Mexico, which seems to be the epicenter of the outbreak.

Swine influenza, or flu, is a contagious respiratory disease that affects pigs. It is caused by a type-A influenza virus. Outbreaks in pigs occur year-round. The current strain is a new variation of an H1N1 virus, which is a mix of human and animal versions.

When the flu spreads person-to-person, instead of from animals to humans, it can continue to mutate, making it harder to treat or fight off because people have no natural immunity.

The symptoms are similar to the common flu. They include fever, lethargy, lack of appetite, coughing, runny nose, sore throat, nausea, vomiting and diarrhea. he virus spreads when an infected person coughs or sneezes around another person. People can become infected by touching something with the flu virus on it and then touching their mouth, nose or eyes.

The WHO has called the outbreak a "public health emergency of international concern."

Researchers are trying to determine how easily it can jump from person to person. And Keiji Fukuda, WHO's assistant director-general, said it was too early to predict whether there will be a mild or serious pandemic.

The cases confirmed in the U.S. and Mexico were enough of a concern for Andorra Vassiliou, the European Union's health commissioner, to recommend against travel to North America.

People "should avoid traveling to Mexico or the USA unless it is very urgent for them," Vassiliou said.

Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention, said Vassiliou's warning was "not warranted."

In Mexico City, authorities closed all schools and universities until further notice because of the virus, and military troops distributed 4 million filter masks in the city of 20 million residents.

Mayor Marcelo Ebrard said he is wearing a mask "to promote to people to use" it.

"The potential damage for the city is very high," Ebrard said.

The streets of Mexico City were eerily quiet Sunday afternoon -- a time when families are usually out strolling.

Officials have talked about shutting down the bus and subway system, and incoming international passengers at the country's airports are asked on a form whether they have various symptoms that might indicate that they're carrying the virus.

Mexican Finance Minister Augustin Carstens said Sunday that the World Bank was lending his country $205 million to deal with the outbreak.

In Washington, the government declared a public health emergency -- a step Homeland Security Secretary Janet Napolitano said, "sounds more severe that really it is."

"This is standard operating procedure and allows us to free up federal state and local agencies and their resources for prevention and mitigation," she said.

Meanwhile, Israel and New Zealand were investigating unconfirmed cases of swine flu.

Concerns about the virus prompted Canada to issue a travel health notice, and South Korea to say it will test airline passengers arriving from the United States.

Japan is expected to convene a Cabinet meeting Monday to come up with measures to block the entry of the virus into the country.

In New Zealand, officials said 22 students and three teachers, who returned from a three-week-long language trip to Mexico, might have been infected. The group remains quarantined at home, and Health Minister Tony Ryall said 10 students tested positive for influenza A -- the general category of strains that includes the H1N1 swine flu. In Spain, six people -- all recently returned from Mexico -- were being isolated in hospitals, the country's Health Ministry said. Lab tests confirmed that one of the cases had tested positive. And in Israel, doctors are running tests on a man who recently returned from Mexico with light flu symptoms. iReport.com: Do you think we should be worried about swine flu? In 1968, a "Hong Kong" flu pandemic killed about 1 million people worldwide. And in 1918, a "Spanish" flu pandemic killed as many as 100 million people.