The swine flu epidemic entered a dangerous new phase Monday as the death toll climbed in Mexico and the number of suspected cases there and in the United States nearly doubled.
California and Texas confirmed new cases, and New Jersey and North Carolina joined the growing list of states with suspected swine flu cases.
The World Health Organization raised its alert for the new strain of swine flu by one level, two levels short of declaring a full pandemic.
"The change to a higher phase of pandemic alert indicates that the likelihood of a pandemic has increased, but not that a pandemic is inevitable," the agency said in a statement.
Swine flu is new flu virus combining strains of human, bird and swine flu. Despite the name, experts believe the virus is being transmitted human-to-human, not pig-to-human. The USDA says it is safe to continue eating pork, however.
On Monday, there were several new cases of swine flu in the U.S. including 28 in New York, 13 in California, 6 in Texas, 2 in Kansas and 1 in Ohio, raising the total number of cases nationwide to 50.
The most severe case reported so far in California was that of a 35-year-old woman hospitalized in intensive care before recovering.
For the first time, North Carolina reported suspected cases of swine flu, but investigators declined to say how many cases or where they were located.
New Jersey health officials say they have identified five probable cases of swine flu in people who recently traveled to Mexico and California.
Meanwhile, Texas officials confirmed at least three cases of swine flu. Most of the confirmed cases involve students at Byron Steele High School in Guadalupe County near San Antonio, said Carrie Williams, spokeswoman for the Texas Department of State Health Services.
Texas authorities have ordered closed all 14 schools and two district facilities in the Schertz-Cibolo-Universal City Independent School District until further notice.
A student of Canyon Creek Elementary School in Richardson, Texas has also reportedly tested positive for swine flu, prompting authorities to close the school for the rest of this week. Two other students at the school are suspected of being infected at this time.
Dr. Jeffrey Engel, Texas health director, said officials are involuntarily isolating patients who may have the virus. But he said the handful of cases are sporadic and in different parts of the state.
Investigators are gathering specimens and hope to know whether the cases are "probable" some time Tuesday and will seek confirmations by Wednesday.
Engel said the suspected cases are related to travel, mostly to Mexico. He said nobody has been hospitalized and that the suspected cases have been ordered to in-home isolation.
Internationally, swine flu is believed to have killed as many as 149 people in Mexico. Late Monday night, officials confirmed the first suspected case in the country was detected in the southern state of Oaxaca. However, Health Minister Jose Cordova said it was too early to identify the cause of the geographical source of the disease. Elsewhere, there have been 2 confirmed cases in Scotland and one in Spain. Health authorities say a South Korean citizen is being tested for possible swine flu.
The government's disease control center said in a statement Tuesday that the person is one of three people reported to have swine flu symptoms after trips to Mexico. New Zealand has also announced at least one suspected case.
WHO's assistant director-general Keiji Fukuda added that "at this time containment is not a feasible option" and the global health body is instead urging affected countries to mitigate any outbreaks.
Fukuda said the evolving crisis could easily push the pandemic alert higher or lower in the coming days.
WHO is not recommending border closures or travel bans, Fukuda said.
"At this time instituting travel bans would really not be very effective" as the virus has already spread to several other countries, he said.
But he said people who are ill should delay international travel, and those developing symptoms after international travel should seek medical attention.
Twenty-eight of the 44 confirmed cases are clustered at a single private school in Queens, N.Y. Another 17 cases of swine flu are suspected in New York.
The head of a New York City high school says one of the 28 people who have confirmed cases of swine flu at the school is a teacher.
Brother Leonard Conway of St. Francis Preparatory in Queens said Monday that 10 teachers are experiencing flulike symptoms.
He says that 160 students have called out sick since last week and that parents have called about other new cases.
New York City Mayor Michael Bloomberg said all of students from St. Francis Preparatory in Queens, the largest Catholic high school in the U.S. with 3,000 students, are recovering.
Mexico is closing all schools nationwide until May 6 as a precaution.
Mexico Health Secretary Jose Angel Cordova said only 20 of the 194 suspected deaths have been confirmed to be from swine flu and the government was awaiting tests results on the rest.
He said 1,995 people have been hospitalized with serious cases of pneumonia since the first case of swine flu was reported on April 13. The government does not yet know how many were swine flu.
Of those hospitalized, 1,070 have been released.
The U.S. Centers for Disease Control and Prevention are on high alert tracking swine flu cases throughout North America and the world.
The U.S. government declared a public health emergency Sunday to respond to the outbreak.
EU health officials urged Europeans on Monday to postpone nonessential travel to the United States and Mexico because of the swine flu virus.
Related: Europe, Asia Brace for Swine Flu
Singapore, Thailand, Japan, Indonesia, and the Philippines dusted off thermal scanners used during the 2003 SARS crisis and were checking for signs of fever among passengers arriving from North America. South Korea and Indonesia introduced similar screening.
China, Russia and Taiwan said it would quarantine visitors showing symptoms of the virus amid a surging global concern about a possible pandemic.
The World Health Organization spokesman Paul Garwood said the health body was recommending calm and common sense — "if people feel sick, if people feel they are suffering from some kind of ailment like flu (then) they need to go and see a doctor."
"There are measures in place that can treat this illness," Garwood added.
Flu Activity in the U.S.
While 44 people have been diagnosed with swine flu in the U.S., none of the cases has been fatal.
President Barack Obama said Monday the spread of swine flu is a cause for concern but "not a cause of alarm" and he's staying on top of the problem.
Obama told a gathering of scientists Monday that the administration is "closely monitoring" cases of swine flu, how many people have it and what the threat is. Obama also said the American people can expect to get regular and frequent updates about what Washington is doing.
He said the swine flu threat dramatizes how the United States cannot allow itself to fall behind in scientific and medical research.
The acting head of the CDC, Dr. Richard Besser, said Monday U.S. officials were questioning border visitors about their health.
Gov. Arnold Schwarzenegger said California is prepared for swine flu.
The governor told a Beverly Hills press conference Monday that the number of confirmed cases in the state stands at seven — four in San Diego County and three in Imperial County.
Schwarzenegger says all have recovered.
The governor says California has activated its joint emergency operations system — a combination of the public health department and the state emergency management agency.
Schwarzenegger says flu experts have been deployed to Southern California and the state is communicating with public health officials in Mexico, where the situation is much worse.
Treating Swine Flu
Besser said Monday people can best protect themselves against the swine flu threat by taking precautions they were taught as kids, like frequently washing their hands and covering their mouths when coughing.
He said there is no reason for Americans to begin wearing face masks as residents of some Mexican states are now doing.
There is no vaccine available to prevent the new swine flu. However, there are antiflu drugs that do work once someone is sick.
Bresser said the government is still deciding whether to order that a swine flu vaccine be produced. He said vaccine manufacture takes a long time and would interrupt ongoing work to create next winter's regular flu shot.
But if a vaccine eventually is ordered, the CDC already has taken a key preliminary step — creating what's called seed stock of the virus that manufacturers would use.
The virus appears responsive to the antiviral drugs, like Tamiflu and Relenza, which can be used to reduce the severity of the flu if used within two days of the appearance of symptoms.
Dr. Robert A. Salata, chief of the Division of Infectious Disease at University Hospitals Case Medical Center in Cleveland, told FOXNews.com that the fact that the severity of the virus can be weakened by antiviral drugs is encouraging.
"This past flu season, all of the influenza type A viruses like this one, were resistant to Tamiflu," said Salata, who is also a professor at Case Western Reserve University. "These viruses appear to be sensitive to all four antiviral drugs on the market."
Roughly 12 million doses of Tamiflu will be moved from a federal stockpile to places where states can quickly get their share if they decide they need it, Homeland Security Secretary Janet Napolitano said.
Salata said people should be cautious, but not panic.
"It’s our responsibility to educate people on what they’re risks are and to encourage them to try to take measures to prevent the spread of the disease," he said. "Certainly people should stay away from people who are know to be ill, practice good hand hygene, as well as good cough and sneeze etiquette."
Drugmaker Baxter International Inc. has requested the swine flu virus sample from the World Health Organization so that it can research the virus and then develop a vaccine in what the company spokesperson said is half the time, approximately 13 weeks, of normal manufacturing, which is usually 26 weeks. Baxter specializes in research and development in emerging vaccines.
Esti Lamonaca's illness started with a high fever, a cough and achy bones, just a couple of days after she returned from a spring break trip on the beach in Cancun with friends. By the weekend, her voice was hoarse and she was wearing a surgical mask.
Officials at Lamonaca's school, St. Francis Preparatory in Queens, whose notable alumni include baseball's Joe Torre and football's Vince Lombardi, as well as TV personality Julie Chen, learned that something was wrong there on Thursday when students started lining up at the nurse's office complaining of fever, nausea, sore throats and achy bones. It wasn't long before the line was out the door.
The nurse notified the city Health Department that day. On Friday, more students were getting sick, and the department dispatched a team to the school at about 1:30 p.m. But they got caught in traffic and didn't arrive until 3:30 p.m, just as classes were letting out for the weekend, said Brother Leonard Conway, the school's principal.
By then, there were only a few students left, and health officials quickly tested them for swine flu. As many as 150 students are suspected to have been infected. Officials think they started getting sick after some students returned from the spring break trip to Cancun, although Mexico officials have yet to turn up any cases in Mexico.
Cleaning crews spent Sunday scrubbing down St. Francis, which will be closed for days.
"I haven't been out of my house since Wednesday and am just hoping to make a full recovery soon," Lamonaca said. "I am glad school is closed because it supposedly is very contagious, and I don't want this to spread like it has in Mexico."
Some schools in Texas, California, Ohio and South Carolina also were closing after students were found or suspected to have the flu.
The outbreak has people on edge across the country.
In Ohio, a 9-year-old boy was infected with the same strain suspected of killing dozens in Mexico, authorities said. The third-grader had visited several Mexican cities on a family vacation, said Clifton Barnes, spokesman for the Lorain County Emergency Management Agency.
"He went to a fair, he went to a farm, he went to visit family around Mexico," Barnes said.
The boy has a mild case and is recovering at home in northern Ohio, authorities said.
His elementary school in Elyria was closed for the week.
Officials along the U.S.-Mexico border asked health care providers to take respiratory samples from patients who appear to have the flu. Travelers were being asked if they visited flu-stricken areas.
In San Diego, signs posted at border crossings, airports and other transportation hubs advised people to "cover your cough." At Los Angeles International Airport, Alba Velez, 43, and her husband Enrique, 46, were wearing blue face masks — purely as a precaution — when they returned from a trip to Mexico.
The Los Angeles couple hadn't seen anyone sick while in Guadalajara but were nervous because of the stream of information about new cases. The two were wearing the masks because they're "just cautious," Enrique Velez said.
It was a different story for travelers heading south of the border.
"I'm worried," said Sergio Ruiz, 42, who checked in for a flight to Mexico City after a business trip to Los Angeles and planned to stay inside when he got home. "I'm going to stay there and not do anything."
In New York, Jackie Casola — whose son Robert Arifo is a sophomore at St. Francis — said her son told her a number of students had been sent home sick Thursday and hardly anyone was in school Friday.
Arifo hasn't shown any symptoms, but some of his friends have, his mother said. And she has been extra vigilant about his health.
"I must have drove him crazy — I kept taking his temperature in the middle of the night," she said.
The Associated Press, Reuters and FOXNews.com's Marrecca Fiore contributed to this report.