Influenza, commonly called the flu, is a viral respiratory infection that spreads from person to person through coughing, sneezing, and contact with contaminated surfaces.
In colder climates, influenza is seasonal. In the United States, flu season can begin as early as October and end as late as May. In warmer regions of the world, it may be present year-round. During each flu season, there are multiple strains of influenza present, but typically one or two strains predominate as that year's "seasonal flu."
Influenza can cause a spectrum of illness, from mild to severe, and sometimes it can be fatal. According to the Centers for Disease Control and Prevention (CDC), millions of Americans get the flu in the U.S. each year and thousands of people are hospitalized. Seasonal flu-related deaths are difficult to estimate and vary from year to year. (See the CDC’s FluView for the latest estimates.)
With the usual seasonal flu, the highest infection rates are seen in the very young, women who are pregnant, and people with weakened (compromised) immune systems or pre-existing lung disease, but the elderly are especially vulnerable. Most influenza-related deaths and hospitalizations occur in those over 65 years of age.
Estimating the actual number of flu cases is difficult because many of those who get the flu do not seek medical treatment and, of those who do, only a small number are tested. Testing is more common for people who are hospitalized, but overall, laboratory-confirmed cases of influenza only represent a small percentage of those in a community who actually have the flu.
Influenza A and B viruses change over time; they can mutate (antigenic drift) to develop different strains. Seasonal influenza strains can undergo a series of genetic changes (antigenic shift) so that people no longer have immunity from prior infections or vaccination. Usually a single strain of influenza virus A will predominate during a particular flu season, although there may be a mixture of A and B causing outbreaks in the community at the same time.
When a large number people are susceptible to a virus, an influenza epidemic may occur. Of particular concern is the spread of a virus globally, resulting in a pandemic. Influenza A can undergo a major genetic change that can make a virus strain much more lethal and/or easier to transmit.
Flu vaccines that are developed each year to prevent flu infection are based upon data as to which strains are likely to be most common that season. They usually contain weakened forms of the virus (attenuated) or inactivated virus targeting two influenza A strains and one influenza B strain.
Different strains of influenza viruses cause illness in humans and in many animals, including birds, pigs (swine), dogs, whales, horses, and seals. Human influenza strains pass easily from person to person, but most strains of animal influenza rarely infect people. When they do, it is almost exclusively when there is significant close animal contact, such as a person who raises chickens or pigs, or at state fairs. Those infections do not usually pose a risk for person-to-person transmission.
The ongoing worry of the world's medical communities is that an influenza strain that is infecting animals, such as birds or pigs, will mutate sufficiently that it will cause serious illness and death in humans (who have no protective antibodies against it) and that it will become a strain that is transmitted easily from human to human.
For example, during the 2009-2010 flu season, there was a seasonal influenza A strain as well as a new strain identified as 2009 H1N1 influenza A (sometimes called swine flu). According to the CDC, this new strain caused the majority of influenza infections in 2009-2010, and the World Health Organization (WHO) declared the 2009 H1N1 virus a pandemic since it caused multiple influenza outbreaks throughout the world at the same time.