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I can understand that our immune system may be weaker when we are cold (?), but since these viruses live inside a human body which internal temperature is constant over the year, why does the season make a difference?

Moreover, we are never sick from the flu in summer, like it does not exist, yet the viruses reappear as soon as the cold season is back. If nobody was sick before winter, where are the viruses coming from?

Do people with a constantly weakened immune system (e.g. AIDS) catch flu in summer too ?

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This is a really interesting question, because it really isn't as easy as people being a bit more susceptible in the winter. There's no definitive answer yet.

Influenza Seasonality: Underlying Causes and Modeling Theories says:

In temperate climates, flu infections at whatever level of intensity are characterized by a flu season. In these areas, the disease is thought to exist at a low level throughout the year but exhibit a marked seasonal increase, typically during the winter months. Influenza epidemics and outbreaks occur in tropical areas as well, although the timing and impact are not as well defined

The exact reason why this flu season happens are not known, but a few things come together that are good for influenza:

Things that are debated:

  • Crowding and being inside more might be a factor, with people being closer together because they spend a lot of time indoors.

As for the second question:

If nobody was sick before winter, where are the viruses coming from?

Well, there's a whole different hemisphere "down" from where you probably are, where they have the flu season while the Northern Hemisphere is mostly healthy :) Air travel is convenient for influenza. Even without it, we'd still have influenza during the winter, though - as mentioned above it does occur in tropical areas and can spread from there. Also, humans aren't the only species to carry around influenza - wild birds are one of the primary sources of influenza.

Do people with a constantly weakened immune system (e.g. AIDS) catch flu in summer too?

Well, everyone can - it's unlikely, but it does happen. And as flu season passes, there just aren't many people around that you can catch it from.

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    Thanks ! I especially liked the examples and references to research papers. – JulienD Mar 23 '16 at 22:38
  • Regarding summer flu -- I came down with influenza A early last August. I was told by the clinic that they had seen cases in July. The irony was I had planned to get a flu shot the next week. – Carey Gregory Mar 23 '16 at 23:46
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    @CareyGregory that's early for a flu shot! So far, I have never had influenza, just colds (summer and winter), but I can imagine that are also not particularly pleasant when it's hot outside. – YviDe Mar 24 '16 at 11:11

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