The incidence of Influenza ('flu) is tightly linked to the seasons. In temperate climates, it's thought that the disease exists at low levels during the warmer Summer months, and then dramatically peaks during the Winter.

The reasons for this 'seasonality' have been examined and discussed at length for more than a century.

There are dozens of different theories, none of which has been accepted as the cause. It's likely that the Winter increase is due to a variety of factors - but there is currently no agreement on which are the important ones, or how they might be interacting.

While epidemiologists, virologists, immunologists, and mathematicians have all developed laudable theoretical and empirical models to explain seasonality, none are so complete as to fully and adequately explain the phenomenon.“
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Theories include the possible influences of :

• Changes in ventilation (as more people tend to gather inside buildings during the winter)
• The general health of the population, which tends to be lower in Winter (due to lack of sunlight, poorer diet etc) leading to greater chances of being infected.
• The ambient temperature which somehow affects the virus replication.
• The possibility that indoor heating somehow increases the virus potential.
• Increased air travel aiding the spread of the virus.
• 'Bulk aerosol transport' (.i.e. movements of large amounts of air containing the virus)
• Global weather systems - such as El Niño.

For more details see : Influenza Seasonality: Underlying Causes and Modeling Theories Journal of Virology, Vol. 81, No. 11, 2020

The myriad theories accounting for seasonality reviewed in this paper, as well as those that will hopefully emerge as influenza continues to rise in prominence, suggest that the elegant and predictable periodicity of nonpandemic influenza is caused by a less-than-straightforward interaction of many different factors.”
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Note:

It's likely that many of the poorly understood mechanisms which affect the seasonal incidence of 'flu will also be important factors for the current Covid-19plugin-autotooltip__plain plugin-autotooltip_bigCovid-19

Editor's note : Because of the intense, ongoing, global research efforts related to Covid-19, it's difficult to get an accurate picture of the unknown factors. Here are some of the many as-yet-unanswered questions Why does the virus have much more serious eff…epidemic.

In addition, in many countries, the latest Winter figures for the number of 'flu cases has recently dramatically fallen ^{[ref. needed]}. It's assumed that the drop is related to behavioural changes linked to the Covid epidemic.

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