Why is it that you get colds or flu more commonly in the winter than summer? Why do they only give out the flu vaccine in the winter time?

It may seem like they occur more often in winter but actually epidemics can and do occur at any time of year. BUT... when they occur in winter when it is cold and the immune system is depressed (especially in the young and elderly) then they tend to be associated with respiratory problems such as bronchitis and pneumonia and thus cause much more serious diseases and sometimes death. It is for that reason that the flu jab is given in the autumn/winter to the elderly and those at risk (eg with pre-existing lung problems or who are immunocompromised).

Last edited by David Wynick (15th Mar 2008 08:32:35)

Just to add to this, although you can catch the cold/flu virus at any time of year, they really ARE more common in the winter months.
In winter, people tend to spend more time indoors in close proximity to other people than they do in the summer months. Also,  in winter, many people who would normally cycle/walk from place to place opt to use public transport instead. This close proximity, in warm, humid environments, increases the opportunity for the transmission of viruses.

To go further, not only cold/flu viruses but most respiratory pathogens show a strong seasonality in their prevalence, ie bacterial pathogens or viruses transmitted by oral routes (rubella).
Currently there is no satisfactory evidence on why the epidemics happen in winter, all over the world, in relatively different weather. There are multiple factors in play. Crowding is probably be one of them. There is also an identified increase in suceptibility, at least in mice: mice are more susceptible to experimental infection by respiratory pathogens in winter (and in the morning for that matters). Flu virus has also been shown to be more stable in cold fresh air, which might favor transmission in winter. The relationship respiratory viruses with animal host is also incompletely understood. For instance, Aquatic birds are infected by all the A type flu viruses and can transmit them to humans, which has led to suspect that duck farms could represent a reservoir of flu viruses.
The leading theory is that there is a low level infection of the population that starts an epidemic when the conditions are right. However, the complete picture is not yet drawn.