What is it?
Swine influenza is a common and sometimes fatal respiratory disease among pigs, first identified in 1930, that is caused by a Type A influenza virus.
Normally the disease is specific only to pigs. But sometimes pigs can harbour more than one flu virus at one time, which enables the pathogens to mix genes. As a result, a new viral strain emerges that can cross the species barrier to humans, starting with people in contact with infected pigs.
The latest threat is a strain of the H1N1 type of flu virus.
Why the alarm?
The World Health Organisation (WHO) says the virus can be contagious among humans in close contact and the outbreak has "pandemic potential," meaning there is risk of a spread across regions or continents.
In the past century, novel flu viruses have killed tens of millions of people and cost billions of dollars in economic costs. Worries about a new pandemic have focused in recent years on the H5N1 strain of bird flu, which has killed around 250 people since 2003, mainly in Southeast Asia. But H5N1 is hard to transmit among humans, and its threat has been contained by culling infected poultry.
What about the unknowns?
Experts insist there is no certainty that a pandemic will happen or if so that it will be a mass killer. There are many unknowns about the new strain, especially how easily it spreads between people, how virulent it is or could become. Figuring this out will be the work of gene scientists and epidemiologists.
Which countries are most affected?
Mexico is the epicentre of the outbreak, with 103 confirmed and suspected deaths as of Monday and about 400 people hospitalised. In 10 other countries, there have been 57 confirmed or suspected cases, none of them fatal, among people returning from Mexico.
The United States has had 20 confirmed cases, Canada six confirmed cases and Spain one case. Several countries from Colombia to New Zealand are investigating suspected cases.
How does the virus spread?
Swine flu is thought to spread like typical flu, i.e. in viral particles expelled in coughs and sneezes that are then breathed in by someone nearby, or deposited on surfaces that are then touched by the hand and transmitted to the mouth, nose or eyes.
People with the virus may be able to infect others beginning a day before symptoms develop, and up to seven days or more after becoming sick. Young children may be contagious for somewhat longer.
What are the symptoms?
Sudden fever above 38?C, cough, headache, aching joints, nasal congestion, general fatigue and lack of appetite. Some people who have contracted the virus report runny nose, sore throat, nausea, vomiting and diarrhoea. In past cases, swine flu has caused pneumonia and respiratory failure and worsened chronic medical conditions.
Have there been outbreaks in the past?
From December 2005 through February 2009, only 12 cases of swine flu were reported in the United States. In 1988 a pregnant woman died after contact with sick pigs. In 1976, swine flu at an US military base at Fort Dix, New Jersey killed one soldier. Four were hospitalized with pneumonia. At first, experts feared the strain was related to the Spanish Flu of 1918, which killed millions, but the strain never spread beyond the base.
What treatments are available?
The US Centres for Disease Control and Prevention (CDC) recommends oseltamivir (marketed as Tamiflu) and zanamivir (marketed as Relenza) for treating or preventing infection. These drugs work by preventing the virus from reproducing. Most of the previously reported swine flu cases have recovered fully without requiring medical attention and without antiviral medicines.
Is there a swine flu vaccine?
There is a vaccine for pigs, but not for humans. It is unclear whether current "seasonal" vaccines, designed to combat smaller genetic shifts in major strains of flu virus that are in circulation, may provide a shield.
What are the preventative measures?
Public health authorities in many countries have installed classic control measures, screening points of entry and isolating people suspected to have fallen ill. Mexico has ordered the closure of schools and cancelled public gatherings.
Individuals can wear a face mask, avoid greeting someone with a kiss or a handshake, wash their hands frequently and clean commonly-touched surfaces such as telephones, door handles, tables and lift buttons.
Can swine flu be caught from eating pork?
No. The virus is respiratory, and not transmitted by food. Cooking pork to an internal temperature of 160?C kills viruses and bacteria.
Sources: WHO, the US CDC, European Centre for Disease Prevention and Control (ECDC), French ministry of Health.