The next time you cough or sneeze, there is good reason to cover your mouth.

As many as 20,000 viruses are expelled in an average cough, which may be sufficient to infect many people - particularly those who are not vaccinated.

Julian Tang, a consultant at Singapore's National University Hospital's Division of Microbiology, said as many as 3,000 tiny droplets are produced in a standard cough.

Using previous research on influenza viral loads in nasal secretions, and assuming that each coughed airborne droplet measures between 1 to 5 micrometers in diameter, that adds up to many, many viruses in a single cough.

"Based on this research and assuming about 3,000 droplets are produced per cough, this range of influenza viruses produced per cough is about 195 to 19,500," Tang said.

He added that this figure of 3,000 droplets refers to those that remain suspended in the air for considerable periods - long enough to infect people.

" Larger droplets carrying influenza viruses may also be produced during a cough, but these will fall to the ground relatively quickly and will no longer be considered to be significant in the airborne transmission of influenza," he said.

With a new N1N1 influenza virus spreading around the world, governments in many countries have revived advertisements entreating people to observe personal hygiene.

However, experts are uncertain as to what constitutes an infective dose when it comes to influenza viruses.

But as a guide, previous research has found that it takes just one to 10 organisms to cause viral hemorrhagic fevers, and 10 to 100 organisms to cause viral encephalitis.