The symptoms of the flu include the abrupt onset of a high fever, sore throat, dry cough, general body pain, headache, and tiredness. This may last for about three to seven days, but the cough and tiredness may last a while longer. Transmission is by respiratory droplets, and the incubation period is generally one to four days.
Epidemics tend to occur during the winter months in the Northern Hemisphere, and during the summer months in the Southern Hemisphere. Every year, they are a major cause of death among the elderly and the immunocompromised.
Influenza C commonly infects children, but is not a major cause of disease. Influenza A and B both cause disease, and are both capable of undergoing antigenic drift (meaning that they both mutate because of point mutations). Influenza A is also able to drastically alter its genome, because it is capable of undergoing antigenic shift. What this means is that influenza A is capable of undergoing genetic recombination between viral stains. This can, and has, led to major worldwide pandemics.