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Why do we get the flu most often in the winter? Are viruses more virulent in cold weather?

Hugh D. Niall is a medical doctor and Chief Executive Officer at Biota Holdings Limited in Melbourne, Australia. He heads a scientific team that developed a flu drug known as Zanamivir, which is now in clinical trials in the United States. Niall offers the following explanation.
Electronmicrograph of Flu Virus
Image: Centers for Disease Control and Prevention

FLU VIRUS

Every winter in the United States and other countries with largely temperate climates, there is a sharp rise in the incidence of respiratory infections, the milder of which are popularly described as "colds" and the more severe as "flu." These are caused by quite different viruses, but the distinction is blurred by an understandable tendency of some people who have colds to exaggerate the severity of their illness and lay claim to the status of being a victim of influenza.

This means that true "flu" is really a less common but a much more severe illness than many people realize. It nevertheless infects about 10 percent of the population each year. This percentage can rise to 25 or 30 percent in an epidemic year. For comparison, adults in the United States average two to four colds per year, and children six to eight.

Flu is characterized by the quite sudden onset of feverishness, with a sore throat and nasal discharge, chills, headache, muscle aches and loss of appetite, usually with fever of 100 to 104 degrees Fahrenheit. Over the next few days, the general symptoms may improve but the local symptoms (sore throat, cough) get worse. In an uncomplicated case the patient will be much improved after five to seven days but may take up to two weeks or even longer to recover completely. Flu can lead to serious complications, including bronchitis, viral or bacterial pneumonia and even death in elderly and chronically ill patients. Twenty-thousand or more people die of flu in the U.S. each year.

The winter flu epidemic in a given locality reaches its peak in two to three weeks and lasts five to six weeks. Then it disappears as quickly as it arrived. The reason for this is not completely clear. The usual pattern is for a rise in the incidence of flu in children, which precedes an increase in the adult population. Presumably children are infected at school or kindergarten, bring the virus home and infect their siblings and parents. The parents then pass on the flu to their friends and fellow workers, with a second generational leap upwards to the elderly. Nursing home epidemics are common, and that is where most of the serious complications and deaths due to flu occur.

flu graph
Image: Health Care Financing Administration

This vertical transmission model is a reasonable description of events but begs some questions and leaves others unanswered. It does not explain how the virus entered the pool of school-age children in the first place. Of course, children will be more susceptible to infection as they will have, as a group, a low level of immunity to flu from not having encountered it previously. The virus will therefore be able to spread readily. If there is a low level of flu in the community, a school would be a logical site for an explosion of the viral population. If the strain of flu is one that has not circulated for many years, there may be a large enough population of susceptible adults to sustain the epidemic.

Another problem with the vertical upwards transmission model is that it does not explain how flu can break out simultaneously in areas that are far from one another geographically, when there is no apparent possibility of transmission between the sites. One theory is that the virus may be present in a latent form in asymptomatic carriers, who reactivate virus (one wonders why) and infect susceptible contacts.

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.

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