What is swine flu?

The original virus is a contagious respiratory disease in pigs caused by type A influenza. Pigs are hit by regular outbreaks. There are many different types of swine flu and the current human cases involve the H1N1 strain of the type A influenza virus.

The A(H1N1) strain has never before circulated among humans and is not related to previous or current human seasonal flu viruses, the World Health Organisation (WHO) says. The virus is passed from person to person. There are no known cases of humans being infected by exposure to animals.

The WHO says it is not known where the virus originated. The first cases to get publicity were in in Mexico, and a total of 112 countries have since reported nearly 77,201 infected people and 332 deaths. Last month, the WHO declared the world was at the start of a swine flu pandemic – the first official pandemic in 41 years.

Human strains of flu kill several hundred thousand people worldwide every year, but strains of flu virus that originate in livestock such as pigs or chickens have the potential to kill millions worldwide if they start spreading among people. This is because unlike ordinary strains of winter flu, humans have little resistance to them.

Public health experts are particularly concerned about strains that might spread from pigs or chickens to humans because the animals can act as "crucibles", harbouring several flu viruses at once that can swap genes, in the process becoming more virulent.

In the UK, animal health officials insist there is no evidence this outbreak originated in pigs. The only instance of the strain being found in pigs was in Canada where it was linked to a farm worker who returned from Mexico with flu symptoms, according to the UK environment department, Defra.

How do humans catch it?

The disease spreads in the same way as any other flu: when people inhale airborne particles from coughs and sneezes, or by touching a contaminated surface and then touching the nose or mouth.

You can't catch it from eating pork products. In most cases identified so far the infection makes people unwell but is not life-threatening.

What can I do to avoid infection and stop it spreading?

Virus particles can survive for hours on surfaces such as door handles, so it is important to wash your hands frequently with soap and water.

You should cover your mouth with a tissue rather than a handkerchief if you cough or sneeze. If you do use your hands when coughing or sneezing, wash them as soon as possible afterwards. Avoid touching your mouth and nose with your hands or rubbing your eyes.

Should I wear a mask?

There is little point in wearing a mask unless you or someone you are caring for is suspected of having the infection. Masks only work if changed regularly and they get damp easily, becoming an infection risk.

Surgical masks are not designed to prevent virus particles being inhaled but to stop germs spreading from doctors to patients. Masks that protect against viruses must conform to the European standard FFP3. The US equivalent is N95.

What are the symptoms?

The symptoms of swine flu are similar to normal flu and include fever, cough, sore throat, headache, aching limbs, chills and fatigue.

Some patients have reported loss of appetite, diarrhoea and vomiting. Further information about swine flu is available online at the UK government's Directgov website.

Are there effective treatments?

The antiviral medicines oseltamivir (Tamiflu) and zanamivir (Relenza) can reduce the severity of swine flu. The UK has sufficient stocks to provide a total of 33.5m treatment courses.

Is there a vaccine for swine flu?

The seasonal flu jab given to the elderly over the winter months is unlikely to be effective against swine flu, although it works well against other human flu viruses. The WHO is deciding whether to divert resources from manufacturing regular flu vaccine to making swine flu vaccine.

What is the UK government doing to control the spread?

The government is working with the WHO to monitor the situation. Confirmed cases are being treated with antiviral drugs. The UK has agreements with manufacturers to produce a vaccine if the WHO gives the green light.

What do the WHO's pandemic categories mean?

The WHO defines stages in the progression towards a pandemic from level one to six, with level one meaning there are no animal viruses reported to be infectious to people. Level six is a pandemic, meaning an animal virus has caused sustained community outbreaks in two or more WHO regions.

Would it be better to catch it now and gain immunity ahead of the colder months?

The biggest concern for public health experts is that the flu will die down and then return in an altered and more dangerous form in the winter. The one positive side of the rapid spread of infection is that those who get it now may have some degree of immunity. However, public health experts say complications can set in with any flu and it can turn into a worse illness, so people should not seek to deliberately contract the virus.