The flu, or influenza, is caused by viruses. The word "influenza" comes from Italian and means the "influence of the stars." By the 16th century, when the term was coined, people believed the stars influenced the ravages of viruses. Viruses mutate over time, which is what makes fighting them so difficult. A strain of flu that appears this year may disappear or mutate into something else by next year.
H1N1 (Swine Flu)
The H1N1 strain of influenza first appeared in early 2009 and spread to the United States by April of that year. The World Health Organization declared it a pandemic later in 2009. The name was changed from swine flu to H1N1 when research showed the virus strain was not the same type that normally appears in pigs, the U.S. Centers for Disease Control and Prevention reports. This strain of influenza has been deadly, especially in Mexico where it first appeared. The CDC states that high-risk individuals, such as the elderly, pregnant women and children who are 5 or younger, are more likely to get severe cases of H1N1. The symptoms for most people are similar to those of regular flu, including headaches, chills, a fever or cough, nasal congestion, and fatigue.
H5N1 (Bird Flu)
Avian influenza, also known as bird flu and officially as H5N1, moved from birds to humans beginning in the latter part of 2003, particularly in Asia. The virus is spread to humans when they come in contact with bird feces, saliva or nasal secretions. The H5N1 strain of influenza has been particularly deadly to both birds and humans; bird flu kills more than half the people it infects, according to the Mayo Clinic. The Mayo Clinic reports that health officials are concerned that if the H5N1 virus mutates, it could infect people throughout the world, causing a pandemic.
Spanish Flu
Spanish flu got its name because it killed many people in Spain before spreading around the world. It was a particularly deadly strain of influenza that infected one-fifth of the world's population between 1918 and 1919, according to the Stanford University Human Virology website. It spread so far largely because it appeared as World War I ended and soldiers who were in Europe were going home, many carrying the virus with them. Stanford estimates that between 20 million and 40 million people died from it. The virus also was unusual in that it was more deadly for people between the ages of 20 and 40 than for small children or the elderly.
