[Influenza, an existing public health problem].

Seasonal influenza is an acute and recurring respiratory disease known since ancient times, occuring, in particular, during winter months and having an elevated effect on public health worldwide. The disease has high morbidity rates for people of all ages and particularly high mortality rates for children, adults over 60 years old, patients with chronic illnesses and pregnant women. Prevention control strategies include vaccination using inactivated, subunit or genetically modified virus vaccines. Influenza in humans is caused by two subtypes of influenza virus A and one of influenza virus B. The influenza virus A that affects humans mutates easily, thereby often causing new antigenic variants of each subtype to emerge, requiring the inclusion of such variants in annual vaccines in order to assure proper immunization of the population. The influenza pandemic refers to the introduction and later worldwide spread of a new influenza virus in the human population, which occurs sporadically. Due to the lack of immunity in humans against the new virus, serious epidemics can be provoked resulting in high morbidity and mortality rates. Historically, influenza pandemics are a result of the transmission of the virus from birds to humans, or the transfer of such genes to seasonal influenza. Wild waterfowl--both migratory and shore birds--carry a large diversity of influenza virus subtypes, which are eventually transmitted to domestic birds. Some of those viruses cross the species barrier and infect mammals, including humans. The adaptation process of the avian virus to mammal hosts requires time. Therefore, the presentation of these cases can take several years. Since December 2003, in several Southeast Asian countries a large proportion of domestic birds have been affected by an avian influenza epidemic (subtype H5N1). By Februrary 2006, the epidemic had already affected countries in Europe and Africa, having a significant economic impact on commercial poultry due to the more than 180 million birds that were sacrificed. Some strains of this avian influenza virus have directly, although incipiently, infected the human population. The virus has not yet acquired with complete efficiency person-to-person infection and transmission, which has limited its spread among humans. Since the mortality rate in infected individuals is greater than 50%, the World Health Organization (WHO) called on their member countries to establish preparation and emergency plans against the threat of a possible pandemic associated with H5N1 virus, or another virus related to common influenza. These actions are intended to prevent or reduce the impact of the threat, as experienced in previous pandemics, such as in 1918 when roughly 40 million people died worldwide. The prevention and control plans include, among other strategies, vaccination and antiviral medications. Nevertheless, to date there are no vaccines to be administered to the population in the case of a new influenza pandemic emergency and it is possible that countries that produce the annual seasonal influenza vaccine lack the capacity to produce the pandemic virus vaccine in a short period of time. In addition, recent studies have identified the existence of influenza virus strains resistant to common antiviral agents. The purpose of this review is to update the basic concepts of influenza in order to strengthen epidemiological surveillance of the disease and reinitiate prevention and control actions in the event of a pandemic.