Why does the flu only happen in winter?

Question

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 ?

Answer 1

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:

• Dry, cold air favors transmission - especially dry air is good for it, both for transmission and survival on, for example, surfaces. Heating during the winter decreases humidity inside, while the cold air outside is always less humid.
• Vitamin D is thought to help against infection - during the winter, vitamin D production by the body is lower.

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.

Answer 2

Well, since you already have a well-written answer, I just wanted to add this as another possible cause:

Research done by the National Institute of Child Health and Human Development (NICHD) in 2008 found that the influenza virus has a butter-like coating. The coating melts when it enters the respiratory tract. In the winter, the coating becomes a hardened shell; therefore, it can survive in the cold weather similar to a spore. In the summer, the coating melts before the virus reaches the respiratory tract.

Source: https://zenodo.org/record/1233359#.Xr9iDmnhUzQ

Really cool question, by the way!