About the Flu: Causes
There are three families of influenza virus: A, B and C. Type C is basically a disease of ducks, geese, turkeys, and chickens. It can be involved in a small percentage of human common cold illnesses but is definitely a minor player in human infectious diseases. Type B long ago made a permanent leap to humans. It changes very little from year to year, and tends to cause milder disease.
Type A influenza poses the most serious problems for humans. Strains of this type have been found in birds, humans, horses, pigs, seals, whales, and ferrets. Viruses that affect two different species sometimes combine, mixing-and-matching genetic information, to create a new strain against which nobody is immune and for which no vaccine has been prepared. There are an infinite number of possible new varieties of type A influenza.
Flu takes 1-4 days to incubate in humans, but infected people become contagious before symptoms appear, often after just 24 hours. Adults remain infectious for about 6 days, children up to 10 days. As well as direct spread through sneezing and coughing, viral particles can survive many hours indoors on inanimate objects like telephones and doorknobs and be spread through contact with these.