Influenza, or "the flu," is primarily a respiratory illness. With different categories of Influenza (A, B, or C) the flu is a contagious virus that travels from individual to individual with coughing and sneezing.
Influenza type A can affect not only humans, but pigs, horses, birds and other animals as well. Influenza B is typically a virus found in humans only. Less serious than Influenza A, however, Influenza B is nonetheless dangerous to vulnerable individuals. Influenza C viruses are few and far between and cause minimal symptoms, when compared to the other two families of Influenza.
Most of the more serious flu infections are of the Type A variety. For example, the famed "bird flu" or H5N1 started, to our knowledge, around 1997 with widespread worldwide damages. Over half of those infected with H5N1 have died. Because they were found to be the source of the infection, many diseased birds were annihilated at the beginning of the outbreak.
Another example of a Type A Influenza strain is the H1N1 virus. Like the bird flu, it has quickly spread around the world. Since 2009, it has been infecting many people. That being said, the symptoms are far less deadly, for most people, than the bird flu.
People at high risk for contracting the flu, or at high risk for suffering more severely from the effects, include the elderly, babies and young children, people with compromised immune system function, and those suffering from chronic illness. While most people recover from the flu with a few days’ rest, these vulnerable individuals may suffer complications, like pneumonia, and require treatment in the hospital, including oxygen therapy.