But the frequency of human contact across the world and the highly infectious nature of the virus make this explanation difficult to accept. Moreover, there is no evidence of persistent or latent infection with influenza viruses. In any case, this idea is not really very different from the notion that the virus circulates at a low level throughout the year and seizes its opportunity to cause an outbreak when conditions allow. Even harder to explain is why the flu disappears from a community when there are still a large number of people susceptible to infection.
The answer as to why flu is a winter disease is not fully known. However, flu is spread largely by droplet (aerosol) infection from individuals with a high viral level in their nasal and throat secretions, sneezing and coughing on anyone close at hand. The aerosol droplets of the "right" size (thought to be about 1.5 micrometers in diameter) remain airborne and are breathed into the nose or lungs of the next victim. Situations in which people are crowded together are more common in cold or wet weather--and so perhaps this contributes to spreading the flu at these times. It is interesting that in equatorial countries, flu occurs throughout the year, but is highest in the monsoon or rainy season.
Several recent developments promise to increase our understanding of flu. There are now drugs for influenza (neuraminidase inhibitors) that will potentially treat all strains of this virus; and new tests in development will provide an on-the-spot diagnosis in 15 minutes or less. These advances should lead to flu being accurately diagnosed and treated in the community. This new focus on flu should provide more reliable worldwide data on its incidence and spread.
Until we know much more, flu will remain an unpredictable source of danger to public health worldwide. This fact is well illustrated by the alarming recent identification of an unusual avian strain of flu in Hong Kong in four patients, two of whom died. At the time of writing, this small outbreak is under intensive study by expert groups from the World Health Organization and the Centers for Disease Control and Prevention.