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