Q. Is the risk of swine flu being hyped?

A. No. This is a new flu virus never seen before. It has jumped species from animal to human and has begun to spread from person to person. That is a significant change. As it is a new virus, it has the potential to spread widely because few people will have immunity to it. Although it is the H1N1 sub-type, often seen in seasonal flu, it is a novel strain so people who have had seasonal flu (or the vaccine) are unlikely to be protected.

Q. Wasn't avian flu a new virus?

A. Yes. But it has not spread from person to person (except in a few rare cases). The H5N1 avian flu virus emerged in Hong Kong in 1997 and then again in 2003. It has since infected about 400 people worldwide. All but a handful were infected after direct contact with birds.

Q. Is a pandemic now inevitable?

A. No. The World Health Organisation raised its alert level to phase five (out of six) on Wednesday night, indicating a pandemic was "imminent". It did so because of evidence the virus had transmitted from person to person in Mexico and the US (and in one case in Spain). The virus could spread or peter out – the next few weeks will tell.

Q. If the virus dies away, can we relax?

A. No. We are entering the summer in the northern hemisphere, which marks the end of the winter flu season. The virus is killed by ultraviolet light and spreads less easily when people are outdoors. It could lie low for a few months only to return in the autumn.

Q. Would it be more virulent if it returned?

A. Possibly. Pandemics tend to come in waves of infection, which may be spaced over 18 months or more. The 1918 pandemic came in three waves, each more virulent than the last. Severe flu results in more virus being produced and expelled (in coughing and sneezing) and is thus more likely to be transmitted. As flu viruses are constantly changing (called antigenic drift) the nastiest strain tends to become dominant in the community over time.

Q. Given the uncertainty, why worry?

A. Because preparation is essential. Flu is one of the most infectious illnesses known and can spread fast, infecting tens of thousands in days. Organising our defences, shipping antiviral drugs to pharmacies, setting up telephone and internet flu lines will only save lives if done in time. Making a vaccine is the biggest challenge, and takes four to six months.

Q. What is the difference between swine flu and hayfever?

A. This is the start of the hayfever season and many people will soon start to suffer the familiar blocked nose and raw throat, also be symptoms of flu. The key distinguishing feature is fever. A person with flu will be running a temperature while a hayfever sufferer will not.