PARIS - Don't pick your nose. Wash your hands. Blow your nose with a tissue and dispose of it carefully. Clean door knobs, tables, and elevator buttons. Don't shake hands or kiss. Avoid crowded places. And if you have flu, stay at home if you can.

These time-honoured recommendations of hygiene and social distancing are the central core of defences against any flu pandemic.

"Simple public health measures seem to be highly effective at reducing the transmission of respiratory viruses," according to a study published on Thursday in the British Medical Journal (BMJ), assessing the effectiveness of various methods.

The preferred shield against influenza is a vaccine. But no such formula currently exists for the strain of H1N1 virus that erupted in Mexico and spread to at least a dozen other countries through jet travel.

It is still unclear how easily the virus is passed from person to person, and how lethal it is.

But experts say that, like any flu virus, it will be combatted by a mix of prevention and containment -- in which the role of the individual will be crucial -- as well as treatment.

Flu viruses are transmitted in droplets expelled in coughs and sneezes that are then inhaled by people in close vicinity.

They can also survive on surfaces -- for up to 72 hours depending on the porosity of the surface, local temperature and humidity, scientists have found.

This means that a person can pick up a virus on his fingers and infect himself by touching his nose, mouth or eyes.

Simple hygiene etiquette -- while not a guaranteed shield -- can reduce the risk, say experts.

It means people should not cough or sneeze in other people's faces but use a tissue or, if none is available, a shirtsleeve rather than their hands. They should also wash their hands frequently, using soap and water or an alcohol-based cleaner.

"Hand washing is extremely important," said Didier Houssin, deputy head of the French health ministry. "It's a way of preventing the virus from entering the mouth and respiratory tract."

Commonly-used surfaces should be cleaned frequently. And, where practical, people should put distance between themselves and an apparently infected person -- around six feet (roughly two metres), according to Richard Besser, acting director of the Centers for Disease Control and Prevention (CDC).

Someone feeling sick would be best advised to stay at home and then seek medical care rather than risk spraying their germs around a hospital emergency room or a doctor's surgery.

"If you are sick, if you have something which may be swine flu or any other illness it may be prudent to stay at home until you are better," Keiji Fukuda, World Health Organisation (WHO) assistant secretary general, said on Wednesday.

"This is a way of voluntarily preventing yourself from transmitting infection perhaps to others."

As for wearing a mask, medical opinion is divided as to whether these should be a recommendation for the general public.

In theory, face masks can do a lot to prevent a virus spreading -- providing people wear them properly and do not become over-confident and neglect hand-washing and other recommendations.

A study published in January in the US journal Emerging Infectious Diseases by Australian researchers found that a mask reduced the risk of contracting a virus by more than 80 percent.

The research was carried out among 286 parents in 143 households who had to look after a child with a respiratory illness.

The adults wore either the surgical mask used in hospitals; a so-called P2 mask, designed to filter out water droplets containing viruses; or no mask at all.

After a week, non-mask wearers were four times likelier to catch a cold or flu virus than those who wore the mask. There was no significant difference in the effectiveness of the two masks.

However, the researchers found that more than 50 percent of the people who had been assigned a mask did not wear them properly, meaning they had to put the mask on when they happened to be in the same room as their sick child. The figure of an 80 percent risk reduction comes from those who complied.