More cases of swine flu have been confirmed across the globe, while in Mexico the number of suspected deaths linked to the virus is reported to be more than 160.
John McConnell is Editor of The Lancet Infectious Diseases journal
One person has died in the US and cases have now been confirmed in 12 countries over three continents.
Mexican officials say the spread of the virus is slowing, but international experts are more cautious.
The World Health Organization has raised its pandemic alert to five, the second-highest level, but says it has no immediate plans for another rise.
Here John McConnell from The Lancet Infectious Diseases medical journal answers a selection of your questions about the epidemic.
How can you differentiate swine flu from the common and seasonal flu?
Mvula Zakeyo, Lusaka, Zambia
How do the symptoms of swine, or "novel" flu differ from those of a regular flu, if at all? It would most likely ease widespread panic if people knew what to look for aside of sneezing and nausea.
Adam Cullen, Tallinn, Estonia
Unless you have access to a virology laboratory, you probably can't distinguish the two. Symptoms of swine and seasonal flu include fever, cough, sore throat, body aches, headache, chills, and fatigue. Patients do seem to be reporting diarrhoea and vomiting more often with swine flu than is the case for "normal" flu, but in practice the two forms are probably indistinguishable.
I was surprised to be told that wearing masks can actually increase the risk of getting the infection rather than not wearing one at all. The area around your mouth and nose becomes very warm and moist, which is a perfect breeding ground for the virus and that wearing a mask does not reduce the risk at all because it can filter through the holes. Is this true?
The virus breeds in tissues of the respiratory tract, not on the skin however warm and damp it may get. Surgical masks, as seem to be handed out in Mexico, are designed to prevent transmission of disease rather than prevent its acquisition. Masks will certainly become less effective as they become damp and more porous, so masks would have to be changed several times per day to be of any value. The UK government has not stockpiled masks as part of its pandemic preparedness plan, although there have been reports of the government now ordering supplies of masks.
The Spanish Pandemic killed mainly due to cytokine storms which I understand to be the immune system overreacting. Is the swine flu similar? If so, would drugs to lower the immune system in young adults with the virus help lower mortality rate?
Trebor Evans, Llandudno, North Wales
There is no evidence as yet that the Mexican swine flu virus causes the so-called cytokine storm. I would be very wary about doing anything to dampen the immune response of someone ill with flu.
There is so much publicity on the swine flu. How do we know that the flu has become pandemic at a more local level? What precautions should the general public who do not travel to any infected region take to minimise the risk of infection? Does a rise the temperature contribute to the spread of the virus?
Rozie A R, Sg Buloh, Selangor, Malaysia
We will know the virus is causing local epidemics when there is a continuous chain of transmission from one person to the next within a country. There is no guaranteed way to avoid infection while carrying on the activities of daily life. Employers should instruct staff who do not feel well to not come to work. People who feel ill should stay at home and call their healthcare provider for advice rather than trying to visit the clinic. The flu virus seems to prefer cooler winter temperatures, for reasons we don't understand, hence the annual winter flu seasons.
I am on immune suppressant medication for rheumatoid arthritis. What implications does this have if I come into contact with swine flu?
Susan Seal, Birmingham
You may be more liable to become infected by the virus if you come into contact with it, and the infection may cause more severe illness. You should consult your GP for advice on this question.
Is the virus strain in Mexico stronger than that in the US and elsewhere? Does it have to do with the treatment, immunity, or something else?
Eneida Rista, Tirana, Albania
I wish we knew. The virus does not seem to differ inside and outside Mexico. Access to healthcare may be poorer in Mexico, hence the apparently high mortality rate, but we don't have enough data on illness and death among confirmed cases to even say whether the apparent high mortality rate is "real".
I understand that Mexico City has a population of almost 9 million. Assuming a large proportion of the population has been exposed to the virus isn't 150 deaths rather a small figure, probably surpassed by what we would normally expect as a seasonal outbreak?
John Williams, Bromley
Similar to the answer to Eneida Rista's question above, we don't yet have sufficient data to determine whether the mortality rate is exceptional. What is clear is that the Mexican swine flu virus is a variant of the H1N1 virus new to human beings, is not the same as the seasonal flu viruses that have been circulating this year, and is causing illness and death among human beings.
Why are drugs companies permitted to recommend that workers with seasonal flu take a flu remedy and continue at work when infectious and yet we have a panic over swine flu with isolation? Could the Health Protection Agency work with drugs companies and employers to remind individuals that seasonal flu is contagious, serious and that if you do have flu you must stay at home to minimise risk of transmission? Maybe I'm a cynic but is this not all hyped up to take our collective eyes off the deepening economic situation?
Jean Howard, Street, Somerset
I think the point you are getting at is that drugs against flu are marketed on the basis that they reduce the duration of illness and thus allow earlier return to work. The reason that the NHS does not prescribe Tamiflu and Relenza routinely is that the drugs are expensive and flu is for most otherwise healthy people a self-limiting illness from which they will recover within a week or so. I agree that people who are ill with flu should stay at home and not risk spreading it to work mates or travelling companions. Indeed, if you have flu you will know about it and not want to get out of bed for a couple of days. Anyone who tells you they've had "a touch of flu" has had a cold - flu is far more debilitating than a cold. A flu pandemic will likely deepen the economic recession, so it's not the diversionary tactic I would have chosen were I in government!
I don't understand why this is so important. Only nine people out of hundreds of millions have died from this. Don't more die everyday from regular flu? Aren't there other more serious illnesses that are killing people?
Hope Lomas, Chagrin Falls Ohio USA
Around the world, every day thousands of people are dying of AIDS, tuberculosis, and malaria. AIDS is treatable, tuberculosis and malaria are curable. Diarrhoeal and respiratory diseases, which are almost certainly infectious in origin, kill millions of children under the age of five every year. Even in the fat, wealthy developed world, infectious diseases such as pneumonia (particularly among the elderly) and sepsis (particularly among those with underlying medical conditions) kill hundreds of thousands of people every year. Indeed, overall, infectious diseases cause 15-20 million deaths every year - the cause of about one third of all deaths. Swine flu is notable because it's an exceptional event, something above the everyday toll of deaths caused by infectious diseases. A pandemic similar to that of 1918-19, which killed at least 50 million people, would be a catastrophic event because it would double annual worldwide mortality. However, I think the chances of a repeat of 1918-19 are very small. What swine flu, and the media reaction to it, does highlight is our willingness to accept the "routine" toll of infectious diseases with little apparent concern. However, "Mother dies of TB in Zambia and infects her three children" is not a headline that sells many papers.
I understand that during the 1917 pandemic the first bout of flu occurred in spring, was not too virulent, and died out in the summer. It came back in a more deadly form in autumn/winter. Would anyone contracting the illness in spring be immune later in the year?
Mrs J Renouf, Brighstone, Isle of Wight
A short and sweet answer: Yes.