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 ?