In human beings, it can affect the airways, that is to say, the nose, throat, bronchi, and, less frequently, the lungs. However, it can also affect the heart, brain, or muscles. Flu usually spontaneously heals in a few days, but in some cases, it may worsen due to complications that may turn out to be fatal, especially in little children, pregnant women, senior adults, or people with an impaired immune condition.
Flu globally spreads in seasonal patterns, whether as epidemics or pandemics that cause high morbidity and mortality. Annually, it occurs in autumn and winter in temperate zones. It spreads from infected individuals through spray drops charged with the virus that comes from nasal, bronchial secretion, or saliva that contains any of them when coughing or sneezing or just by talking. It is generally required a short distance (less than one meter) from the sick person to get infected.
In developed countries, yearly vaccination campaigns against flu are conducted for people with a higher risk to get the disease, or that are more vulnerable to its complications, as well as strict controls on poultry. The regular human vaccine is the trivalent vaccine, which contains purified and inactive proteins of the three strains that are considered will be more common in the next epidemic: two strains of influenza subtype A, and one strain of subtype B.6.
A vaccine engineered against seasonal and pandemic influenza of a certain year may not be efficient for later campaigns due to the frequent and fast mutations of the virus (changes in its antigens) and the change of the different dominant strains.