Below is a timeline of the events in the rapidly changing flu outbreak. For answers to frequently asked questions and information on the latest developments, visit ABC News' On Call Swine Flu Center.
Timeline: The Spread of the Swine Flu Virus
Thursday, June 11, 2009WHO alert level: 6
The World Health Organization declares swine flu a pandemic and raises the alert to level 6. It's the first official influenza pandemic in more than 40 years.
The heightened level of alert is based on the geographic spread of the virus, not to its severity. Public health officials say there's no reason to be increasingly alarmed.
However, infectious disease experts say the swine flu outbreak could get far worse later in the fall.
Latest numbers from WHO: 27,000 illnesses, 141 deaths.
Sunday, June 7, 2009WHO alert level: 5
New Orleans Mayor Ray Nagin and his wife are quarantined for three days in Shanghai, China, after a passenger on their flight shows symptoms of the illness.
Thursday, May 28, 2009WHO alert level: 5
U.S. manufacturers this week start receiving sample viruses from which to make swine flu vaccine.
To date, no evidence has been found to link eating or handling pork to contracting swine flu.
Latest Numbers from WHO: More than 13,000 people infected, 95 dead.
Friday, May 22, 2009WHO alert level: 5
CDC says it has one, possibly two, versions of the new swine flu virus that could help drug makers develop a vaccine -- a key early step in the vaccine-making process.
The U.S. government directs $1 billion toward studying the virus and placing orders with five flu vaccine manufacturers. Of that sum, nearly $289 million will go to Novartis, $181 million to GSK, $191 million to Sanofi Pasteur, and $150 million to those companies as well as two others to develop test lots, do clinical trials and test for potency.
"It's critically important that we ask the manufacturers to get started, to begin to make the materials for a vaccine," Dr. Bruce Gellin, director of the National Vaccine Program Office, told ABC News.
CDC reminds Americans not to travel over Memorial Day weekend if they are sick.
An article is published in Science focused on genetic research about how swine flu may have spread undetected among pigs for years.
Wednesday, May 20, 2009WHO alert level: 5
The number of confirmed swine flu cases exceeds 10,000. Cases are present in 40 countries and 80 people have died.
Twenty-six schools in New York City and several others around the country are closed as fear of swine flu spreads one month after it first appeared.
What Happened When: The Swine Flu Outbreak
Tuesday, May 19, 2009WHO alert level: 5
New York City Mayor Mike Bloomberg says the flu is spreading at the Rikers Island jail complex.
New York City's St. Luke's-Roosevelt Hospital Center serves 476 patients in its emergency department -- a new record.
Monday, May 18, 2009WHO alert level: 5
In Japan, the number of cases rose to more than 120 over the weekend. Most of the ill there are teenagers.
WHO's annual meeting begins in Geneva, where the focus is on whether to make a swine flu vaccine and whether to raise to the alert level to pandemic level 6.
Sixteen schools are closed in New York City due to swine flu concerns.
Sunday, May 17, 2009WHO alert level: 5
Mitchell Weiner, a New York City assistant principal, dies from swine flu, becoming the sixth person in the U.S. to die from H1N1 virus. Other U.S. deaths previously reported in Texas, Washington state and Arizona.
Chile confirms its first two cases.
Latest numbers: 8,480 people sick in 40 countries. 75 dead, mostly in Mexico.
Thursday, May 14, 2009WHO alert level: 5
Australian researcher Adrian Gibbs, a scientist on the team that helped develop Tamiflu, says the swine flu virus may be a man-made experiment that mistakenly leaked from a lab. Upon reviewing his report, the CDC says there is no evidence to support his claims. WHO later dismissed Gibb's suggestion.
Latest numbers from WHO: Nearly 6,500 cases in 33 countries, more than 60 deaths.
Saturday, May 9, 2009WHO alert level: 5
Costa Rica confirms its first swine flu death -- the first death outside North America.
Thursday, May 7, 2009WHO alert level: 5
Based on data from previous pandemics, WHO suggests that as many as 2 billion people could be infected if outbreak gets worse.
With the U.S. government working on a reference strain that drug manufacturers can use to make a potential vaccine, health officials consider the best way to administer doses. Government has not yet said for sure that it will proceed with investing in a swine flu-specific vaccine.
Swine Flu Around the World
Wednesday, May 6, 2009WHO alert level: 5
Health officials talk vaccines: WHO says new vaccines wouldn't hit the market for about five to six months; manufacturers say they're well-positioned to produce mass vaccine quantities in case outbreak gets worse.
Last day of daily Homeland Security swine flu briefings unless news warrants. Napolitano says DHS will keep its guard up in the event the virus intensifies in the fall.
Latest numbers from WHO: More than 1,500 illnesses and 30 deaths.
Tuesday, May 5, 2009WHO alert level: 5
Scope of U.S. pork export bans, according to the USDA Foreign Agricultural Service: Seven countries have banned live animals, pork and pork products from all states. Another five have banned pork and pork products from all states. Other countries have banned products from several states -- including China, which has prohibited all live pigs and pork products from 38 U.S. states.
A chartered plane from Mexico arrives in China to pick up its quarantined citizens.
Latest U.S. numbers: 380 confirmed cases in 36 states; First American citizen with swine flu dies.
More than 150 experts meet in Geneva, Switzerland, to assess the virus.
The CDC says keeping U.S. schools closed is not necessary.
Monday, May 4, 2009WHO alert level: 5
Mexican officials say they will allow most non-essential businesses to reopen Wednesday.
Some 300 people in Hong Kong remain quarantined at a hotel after a guest tests positive for swine flu.
St. Francis Prep School in Queens reopens.
Latest numbers from WHO: More than 1,000 cases, 21 countries, 26 deaths.
Sunday, May 3, 2009WHO alert level: 5
The virus has spread to 19 countries, including China. The Chinese government quarantines 70 Mexicans.
Three more people in Mexico die from swine flu overnight. The number of cases in the U.S. jumps more than 25 percent since Saturday night.
Still, health officials say the outbreak may have turned a corner. Mexico's health ministry says it may not be any worse than a normal seasonal flu and says the worst of it has passed.
Saturday, May 2, 2009WHO alert level: 5
Italy and Costa Rica become the 16th and 17th countries with confirmed cases of swine flu. There are more than 650 confirmed cases worldwide.
New developments suggest the virus is not as serious as previously thought. Mexico says that of its suspected cases, fewer than half of the people actually had swine flu. Of the more than 300 people that did, 16 died.
The CDC says is encouraged by the news, but will continue preparing for the worst just in case.
In the U.S., there are 160 confirmed cases of swine flu in 21 states. The majority of them did not travel to Mexico.
Dr. Michael J. Ryan, director of the WHO's global alert and response team, says an increase to alert level 6 -- signaling a pandemic -- could be imminent regardless, explaining that "pandemic" suggests the disease's geographic spread as opposed to its severity.
Despite repeated assurances that pork is safe to eat, concerns grow in Canada after pigs there test positive for the virus and that farm is placed under quarantine.
U.S. Swine Flu Worries Leave About 250,000 Children Out of School
Friday, May 1, 2009WHO alert level: 5
An estimated quarter of a million schoolchildren in the U.S. are out of school. In some cities, officials have closed entire school districts due to a single illness. Besser says a school should only be closed if it has a confirmed case of the flu.
President Obama says the U.S. is still preparing for the worst, but adds that this could end up falling short of a pandemic and run its course just "like ordinary flus."
Head of the CDC's influenza division, Dr. Nancy Cox, says preliminary research suggests the makeup of this virus lacks some key components of the 1918 flu pandemic.
Thursday, April 30, 2009WHO alert level: 5
An estimated 160,000 children in the U.S. are out of school in 14 states, including 80,000 students in Fort Worth due to worries about swine flu.
Vice President Joe Biden veers off message, suggesting Americans should take precautions above and beyond those previously suggested by the government. In an interview on NBC's "Today" show, Biden suggests people should avoid mass transit to guard themselves against the flu. Government officials scurry to correct the record.
In the U.S., the CDC's count rises to more than 100 confirmed infections. Worldwide, WHO reports the number of lab-confirmed cases worldwide rose to 236 in 11 countries, with Switzerland, the Netherlands and Peru the latest to announce cases.
Robert Gibbs, White House press secretary, says a staffer on Obama's recent trip to Mexico apparently carried the virus home. Gibbs said that person never came within 6 feet of the president.
The World Bank in Washington, D.C., says a staff member who traveled to Mexico on business April 14-18 has been "preliminarily diagnosed" with swine flu.
Mexico government orders all nonessential businesses to close. According to Mexico's health ministry, there are a dozen confirmed deaths from swine flu in Mexico, but upwards of 170 deaths there are suspected of having been caused by the virus.
Wednesday, April 29, 2009WHO alert level: Raised from 4 to 5 for first time ever.
WHO's decision to raise the alert level helps mobilize pharmaceutical companies and governments to start manufacturing adequate antiviral drugs and speed up the creation of a vaccine.
Health officials confirm the death of a 22-month-old boy in Texas -- the first swine flu death in the U.S. The boy was from Mexico and had traveled with his family to visit relatives in Brownsville, Texas, on April 4. He developed a fever on April 8, followed by other flulike symptoms.
Homeland Security Secretary Janet Napolitano resists the notion that it's necessary to close the U.S.-Mexico border when pressed by Sen. John McCain. "What we have been told by the experts is that any kind of universal closing of a port would have no impact or very, very little impact at all on the spread of this virus," Napolitano says. "This virus is already in the United States, so any kind of containment theory that you're going to keep it out of the United States is -- is really moot at this time."
Gallup poll shows 78 percent of Americans are not worried about swine flu and only 1 to 3 percent are changing their daily lives because of it.
WHO has confirmed more than 100 cases in at least nine countries. In the U.S., the virus in in at least ten U.S. states, infecting nearly 100 people. WHO confirms eight deaths -- sevent in Mexico and one in the U.S.
With the majority of U.S. cases in New York, New York City health officials say they have stopped trying to test everyone who gets sick, just focusing on those with the most severe cases, meaning that the "confirmed" number of illnesses will fall further and further behind the actual number of people who are sick.
Texas Gov. Rick Perry makes an official disaster declaration, allowing access to further state emergency resources. Fort Worth closes all schools until May 8. State officials postpone all public high school athletic and academic competitions until May 11.
Federal health officials say 25 percent of the U.S. stockpile of antiviral drugs like Tamiflu & Relenza are being shipped to states.
China's Commerce Ministry says the government is providing $5 million in humanitarian assistance to Mexico, for masks, gloves, disinfectants, infrared thermal scanners and other equipment.
Egypt begins slaughtering the country's roughly 300,000 pigs as a precaution.
Tuesday, April 28, 2009WHO alert level: 4
Richard Besser, acting director of the CDC, says, "I fully expect we will see deaths from this infection." More than two dozen new cases of swine flu are reported in the US and the virus spreads to additional states.
President Obama asks Congress for $1.5 billion to fight the swine flu outbreak, build drug stockpiles and monitor future cases. Obama says the money will also help international efforts to control the outbreak.
Cuba announces it is suspending flights to Mexico for 48 hours.
Hog prices nationwide drop to an average of about $59 per 100 pounds of carcass weight Tuesday morning, down from about $62 last Thursday, according to the U.S. Department of Agriculture. Prices typically climb past $70 in late April and May.
U.S. officials say they may abandon the term "swine flu," for fear it's confusing people into thinking they could catch it from pork. A better name for the virus, they say, is "H1N1." WHO indicates it had no plans to try to remove the term "swine" from the flu's name.
Canada joins list of countries advising against nonessential travel to Mexico.
WHO confirms 7 deaths in Mexico.
California Gov. Arnold Schwarzenegger declares a state of emergency to help deal with the outbreak.
Sen. Tom Harkin of Iowa, chairman of the Senate appropriations subcommittee that deals with pandemic preparedness, holds an emergency hearing to address funding for states and federal government. Other congressional committees follow suit and plan hearings for later in the week.
Among responses around the world, South Korea and Ukraine have temporarily banned all pork imports from North America in response to flu concerns. Russia, Hong Kong and Taiwan say they will quarantine visitors showing symptoms of the virus.
Swine Flu: The Start of the Outbreak
Monday, April 27, 2009WHO alert level: Raised from 3 to 4 for the first time since the system was adopted in 2005.
The U.S. State Department issues a travel alert, recommending Americans avoid nonessential travel to Mexico.
The number of confirmed U.S. cases more than doubles. President Obama says swine flu is cause for concern "but it's not a cause for alarm."
Twenty-eight of the 45 confirmed cases are in New York, all confined to students at St. Francis Preparatory School and their relatives. Several of the students had spent spring break in Cancun, Mexico.
Swine flu is now detected in nine countries. European Union's health commissioner warns Europeans to avoid nonessential travel to Mexico and the United States.
Mexico City is largely deserted due to an order to effectively shut the city down for the next 10 days.
Sunday, April 26, 2009WHO alert level: 3
Canada becomes third country to confirm human cases of swine flu.
The U.S. government declares a public health emergency, giving federal health officials authority to take rapid measures -- including authorizing contacts and mobilizing the national disaster system -- to respond to the disease, including allowing the use of unapproved drugs.
Officials confirm 20 Americans have been diagnosed with swine flu and expect more to follow. New York Mayor Michael Bloomberg confirms that eight students at St. Francis Prep are infected with the swine flu.
Besser confirms the virus found in Americans is the same as that in Mexicans.
Officials have released 12.5 million of the nation's stockpile of 50 million courses of Tamiflu, a drug that has shown itself at least initially to be effective against the flu virus. But the officials used cautious language in describing whether it could be contained.
In Mexico, more than a thousand people have been infected; WHO has sent a team of experts there to further study the outbreak.
U.S. airlines such as American Airlines, US Airways, Continental and United waive fees for flight changes to Mexico.
U.S. emergency departments step up efforts to control the virus should it surface.
"It's all hands on deck and we're doing fine," White House Press Secretary Robert Gibbs tells reporters at a special briefing at the White House with the nation's top health and Homeland Security officials.
Saturday, April 25, 2009WHO alert level: 3
Running total of confirmed cases of swine flu in the US: 11, including seven in California.
Two students in San Antonio have recovered who had swine flu, but their high school temporarily closes to keep the virus from spreading.
In New York City, eight students from St. Francis Preparatory School in Queens are designated as "probable" cases of swine flu, though not yet confirmed. More than 100 students are sick with flulike symptoms.
WHO holds emergency meeting, says outbreak has potential to become a pandemic.
Friday, April 24, 2009WHO alert level: 3
A government order closes Mexico City private and public schools after Mexico reports its first lab confirmation of swine flu.
The virus has spread to California and Texas; eight Americans infected.
Some influenza experts worry the never-before-seen hybrid flu strain could spur a pandemic, and health officials are on high alert. Still, "at the early stages of an outbreak there's much uncertainty and probably more than everyone would like," says Richard Besser, acting director of the Centers for Disease Control and Prevention.
People warned to take precautions such as covering their mouths when coughing or sneezing and washing hands frequently.
ABC News' Dan Childs, Lauren Cox, Joseph Brownstein, Joanna Schaffhausen, Ammu Kannampilly, Michelle Schlief, Lisa Stark, Huma Khan, Theresa Cook and Kate Barrett contributed to this report.