If the virus enters the nose, throat or lungs of a healthy individual and begins to multiply, symptoms of the flu appear . It can also be transmitted, much less frequently, by direct contact, for example when a person touches a surface that contains influenza virus – for example the hand of a patient – and subsequently touches his nose or mouth.
Sick people are capable of transmitting the disease from one day before the onset of symptoms to about 3 to 7 days after the onset of the disease . Children can transmit the flu even for more than 7 days after symptoms begin.
On the other hand, since influenza viruses frequently infect animals such as birds, horses or pigs, which can be a source of new subtypes for man. "This demonstrated ability of influenza viruses from animals to jump the interspecies barrier and affect human beings means that the flu cannot be considered an eradicable disease ," says the Spanish Ministry of Health about the disease.
