(CNN) -- The World Health Organization has raised its pandemic alert level in response to the outbreak of swine flu that originated in Mexico, U.S. Homeland Security Secretary Janet Napolitano said Monday.
A man wearing a protective mask reads a newspaper Sunday outside a hospital in Mexico City.
The move from level three to level four on the WHO's six-level threat scale means the world body has determined the virus is capable of significant human-to-human transmission -- a major step toward a flu pandemic, said Dr. Keiji Fukuda, the agency's assistant director-general.
A pandemic is not considered "inevitable," Fukuda said. But Napolitano said the move "does indicate that we have a serious outbreak of swine flu on our hands."
At least 75 cases have been confirmed worldwide, including 40 cases in the United States and 26 in Mexico, the WHO said.
Hundreds more cases are suspected, especially in Mexico, where as many as 149 deaths are thought to have been caused by the virus, the country's health secretary said.
"Sadly, 149 people have died, of which we are working to confirm if they are linked to the swine flu," Mexico Health Secretary Jose Angel Cordova Villalobos said. "The number of cases, unfortunately, will continue to increase."
So far, 26 cases have been confirmed by laboratory tests in Mexico and reported to the World Health Organization. Nearly 2,000 people have been hospitalized and 776 remain in hospitals, Cordova said.
The U.S. government is urging travelers to avoid nonessential travel to Mexico because of the swine flu outbreak, and it has started distributing antiviral medications from its strategic stockpile in response to the outbreak, Napolitano said.
The confirmed cases in the United States have been mild so far but, "Scientists can't tell us right now why this is presenting so severely in Mexico City and not as severely up here," she said.
Dr. Gupta in Mexico
Dr. Sanjay Gupta reports from Mexico on the spread, treatment and prevention of the swine flu.
Monday 10 E.T.
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.
The outbreak is a particular concern because of who it is hitting hard, United Nations Secretary-General Ban Ki-moon said Monday.
"We are concerned that in Mexico, most of those who died were young and healthy adults," he said. 
Watch Mexican officials discuss flu plan »
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, Andorra Vassiliou, the European Union's health commissioner, on Monday urged people "to avoid nonessential travel to the areas which are reported to be in the center of the clusters" of a swine flu outbreak.
Public Health Emergency
According to the
World Health Organization, a public health emergency is an occurrence or imminent threat of illness or health conditions caused by bioterrorism, epidemic or pandemic disease, or highly fatal infectious agents or toxins that pose serious risk to a significant number of people.
At a White House news conference Sunday, Homeland Security Secretary Janet Napolitano said the emergency declaration is standard procedure -- citing that one was declared for the inauguration and for recent flooding.
The EU later said that Vassiliou's remarks were her personal comments and that travel advisories can be issued only by member states and not by the EU itself.
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, 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. 
Learn more about swine flu and how to treat it »
The 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. Fukuda, the WHO's assistant director-general, said it was too early to predict whether there will be a mild or serious pandemic.
In Mexico, authorities closed all schools until at least May 6 because of the virus.
They are considering whether to suspend other public activities but are analyzing what economic effects could result, Health Secretary Cordova said.
Military troops distributed 4 million filter masks in Mexico City, which has 20 million residents. Watch CNN's Dr. Sanjay Gupta on the outbreak in Mexico »
The streets of Mexico City were eerily quiet Sunday afternoon -- a time when families are usually out strolling. Watch how even soccer suffers in Mexico »
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 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 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 people had tested positive. 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. In 1918, a "Spanish" flu pandemic killed as many as 100 million people.
