Influenza (the flu) is a viral respiratory infection that spreads from person to person through coughing, sneezing, and contact with contaminated surfaces. The flu is caused by types A, B, or rarely C influenza virus. The most common cause is influenza A, the viral culprit behind flu pandemics and most epidemics. In colder climates, influenza is seasonal, occurring primarily in the winter. 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 A and B viruses change over time. Seasonal influenza strains can undergo a series of genetic changes so that people no longer have immunity from prior infections or vaccination, which is why annual flu vaccination is recommended for persons at increased risk of severe flu-related symptoms. When a large number people are susceptible to the virus, it can cause an influenza epidemic. In addition to this, 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 medical experts' opinions as to which strains are likely to circulate in the community and usually contain attenuated or inactivated virus targeting two influenza A strains and one influenza B strain.
Influenza viruses cause illness in humans and in many animals, including birds, pigs (swine), dogs, and horses. Human influenza strains pass easily from person to person, but most strains of animal influenza only rarely infect humans. When they do, it is almost exclusively when there is significant close animal contact, such as a person that raises chickens or pigs, and the subsequent infection is only rarely transmitted from the infected person to another person.
The ongoing worry for 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. Once this happens, a new worldwide flu pandemic could spread across the world.
Influenza A virus can be further sub-typed based on two unique protein antigens, H (hemagglutinin) and N (neuraminidase). The most common influenza A viruses currently infecting humans have the subtypes H1N1 and H3N2. New flu virus strains are named according to the place where the virus was first observed, strain number, and year. For instance, the predominant strain during the 2003-2004 flu season was influenza A/Fujian/411/2002 (H3N2).
In most cases, the specific name of the influenza virus is only relevant to the medical community and those charged with influenza surveillance, but in recent years there has been news and focus on first avian (bird) flu and then H1N1 (swine) flu.
Avian (Bird) Flu
One of the most predominant recent avian flu strains, an influenza A, H5N1virus was first described in Hong Kong in 1997. It caused an epidemic in birds in Southeast Asia in 2004, and since then it has caused illness and deaths in birds and in some people in parts of Asia. So far this strain of influenza has remained a bird-to-human infection. In 2013, avian influenza H7N9 was first reported to have infected humans in 2013 in China. Reported cases of human infection have resulted in severe respiratory illness.
H1N1 (Swine) Flu
The 2009 H1N1 flu virus was originally called "swine flu" but it is now known to be a combination of human, swine, and avian flu genes. First reported in Mexico and the U.S., it is a new influenza A, H1N1, virus. It is currently the predominant influenza A virus and is causing influenza infections throughout the world.
So far, the 2009 H1N1 flu appears to have had a greater effect on those younger than 65. Between April and December 2009, about 90% of influenza-related hospitalisations and 88% of estimated deaths were in people younger than 65. However, if infected, very young children, pregnant women and the elderly are at increased risk of severe or fatal flu.
Estimating the actual numbers 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 in people who are hospitalised, but overall, laboratory-confirmed cases of influenza only represent a small percentage of those in a community who actually have the flu.
For comparison with the 2009-2010 flu season, the most lethal pandemic in recent history, the 1918-1919 H1N1 influenza A pandemic, infected about one third of the world's population (an estimated 500 million people) and killed an estimated 20 to 50 million persons, with more than 500,000 deaths in the U.S. alone.