Here is a timeline of events in the swine flu outbreak.
Apr 2
A 4-year-old boy is known to have contracted swine flu, which is caused when the H1N1 strain associated with pigs crosses over to the human population. A community in Veracruz has apparently been protesting against pollution from a large pig farm.
Apr 6
Local health officials in the Mexican town of La Gloria in Veracruz state declare a health alert due to a respiratory disease outbreak. About 60% of the town of 3,000 are affected.
Apr 17
US health agencies say that two children in southern California have illnesses caused by infection from swine flu. Both children became sick in late March.
Apr 24
Mexico's Minister of Health confirms 20 deaths from swine flu, but 40 other fatalities are being probed. Mexico City shuts down schools, museums, libraries, and state-run theatres across the capital.
Apr 25
The Health Protection Agency says it is keeping a close eye on the swine flu outbreak in Mexico and the US.
A British Airways cabin crew member who lands at Heathrow after a flight from Mexico is taken to Northwick Park Hospital in Harrow with "flu-like symptoms". He is later given the all-clear.
Apr 26
Suspected deaths from swine flu outbreak in Mexico put at 80.
World Health Organisation (WHO) director-general Margaret Chan says the outbreak in the country and the United States constitutes a "public health emergency of international concern".
Passengers at Heathrow are kept back on a plane from Mexico City for 45 minutes while they are questioned by health officials.
Scottish Government reveals two people are being treated for suspected swine flu and are in isolation at Monklands hospital in Airdrie, Lanarkshire.
Apr 27
Shares in holiday firms and airlines like Thomas Cook and British Airways tumble on the London market as concerns escalate over the swine flu outbreak and its impact on the travel sector.
Health authorities say a Canadian woman is being treated for suspected swine flu at a Manchester hospital. She is later thought to be clear of the bug.
The Government's Chief Medical Officer, Professor Sir Liam Donaldson warns Britain will be hit by cases of swine flu. Health minister Alan Johnson puts number of suspected UK cases at 25.
Two Scottish holidaymakers to Mexico are confirmed as being the UK's first two cases of swine flu. They are said to be "recovering well" at hospital. Seven other people who have been in contact with the couple are also showing "mild symptoms".
As the suspected death toll in Mexico rises past the 100 mark - with at least 40 suspected cases in the US - WHO raises its alert level for the outbreak from three to four - signifying a "significant increase in risk of a pandemic".
WHO's assistant director-general Keiji Fukuda says of the Mexican outbreak "at this time containment is not a feasible option".
The Foreign Office starts advising British nationals against all but essential travel to Mexico.
Apr 28
The first UK victims are revealed as newlyweds Iain and Dawn Askham, from Polmont near Falkirk. They were honeymooning in the resort of Cancun before returning to the UK on April 21 and falling ill. Officials say they are continuing to "recuperate and recover".
The total of people thought to have been killed in Mexico by the virus reaches 152.
Other countries with confirmed swine flu cases include Canada with six, New Zealand with three, and Spain and Israel with two.
Countries with suspected cases include Brazil, Guatemala, Peru, Australia, and South Korea, and seven EU states.
The Government's Cobra emergencies committee, involving both Mr Brown and Health Secretary Alan Johnson, meet to discuss the swine flu issue.
Mr Brown says Britain is "among the best prepared countries in the world" and adds that the Government was taking "all the urgent action that is necessary" to help prevent the spread of the virus.
WHO officials say the virus could infect up to 40% of the UK population in the next six months if the outbreak becomes a pandemic.
Scottish Health Secretary Nicola Sturgeon says that nine of 22 people in contact with Mr and Mrs Askham since returning home were showing "mild, cold-like" symptoms and were also being tested for swine flu.
UK health bosses unveil plans to distribute leaflets about swine flu to all households across Britain.
The Department of Health also says it is attempting to "urgently increase current stockpiles" of surgical face masks - with some reports putting the total number needed at 32 million.