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.