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.