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.

A micrometer, or micron, is one millionth of a meter, or one thousandth of a millimeter.

"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 told Reuters.

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.

"This (3,000) is also the number of droplets estimated to remain suspended in air for long periods — so-called droplet nuclei. 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 H1N1 influenza virus spreading around the world, governments in many countries have revived advertisements entreating people to observe personal hygiene.

In Indonesia, a slick TV advertisement features a young woman remonstrating a suitor at a village sing-a-long, telling him to get to a health clinic because "that's not a normal cough."

Aimed at the poorly-educated masses, the adverts are set to a dangdut (traditional pop music) theme and feature an easy-to-remember slogan and theme song.

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

Infective Doses

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.

Tang said an infective dose of flu virus would depend on a variety of factors, such as the constitution of people breathing in these droplets and whether they had been vaccinated.

"It is difficult to give an exact number for the infectious dose — and this may even differ for the same individual throughout the year. But probably for immune people, the infectious dose will be higher than for non-immune people — hence, the benefits of vaccination," Tang said.

"Those previously vaccinated or naturally infected to the same or similar virus can develop a rapid antibody response and clear the virus in the respiratory tract before the virus can take hold and cause disease."

"(But) even immune hosts may develop symptoms if the viral load exposure is sufficiently high," he added.

Doctors say most airborne flu transmissions occur within a one meter range, although it can also be transmitted by direct contact with contaminated objects, like toys.

"Wearing masks may well help to reduce the transmission of these infections, as well as covering the mouth with your hand or a tissue when coughing or sneezing — simply by using a barrier to prevent dissemination of the virus," Tang said.