The term "flu" refers to any of several acute, febrile, highly contagious viral diseases marked especially by respiratory or intestinal symptoms. RNA viruses of the family Orthomyxoviridae, commonly known as influenza virus, are held responsible for many flu-like symptoms. The illness comes on suddenly, and the symptoms, including fever, body aches, headache and fatigue, tend to develop quickly and are usually more severe than the typical sneezing and stuffiness of a cold. Currently, two strains of flu, seasonal flu and H1N1 or swine flu, are the major circulating flu viruses in the United States, according to Flu.Gov.
The Flu Viruses
The flu is caused by any of several influenza viruses that infect the respiratory tract. An influenza is normally of three types designated as A, B and C. Two main types of influenza viruses--type A and type B--cause the outbreaks and epidemics of respiratory illness that occur almost every winter. According to Illinois Department of Public Health, "More than 200,000 people are hospitalized from flu complications each year in the U.S." Influenza viruses have high mutation and reproductive rates. Over time the vaccines fail because these mutations accumulate and eventually the virus evolves into a new strain.
H1N1 Swine Flu
H1N1 flu, also called swine influenza, is a contagious flu virus that causes respiratory illness in humans. The first case of swine flu in the U.S. was reported in April 2009. Swine flu includes two genes from flu viruses in pigs from Europe and Asia. It also has avian and human genes. H1N1 flu viruses are spread mostly through close contacts such as coughing, discharge from the nose or eyes or sneezing of infected persons. Illness caused by swine flu can range from mild to severe.
H5N1 Bird Flu
H5N1 is a deadly, virulent strain of bird flu with pandemic potential. It remains a serious threat to human and animal health. This virus is highly contagious among birds and does not commonly infect humans, so there is little or no pre-existing immunity in human population against H5N1 bird flu. The symptoms of the H5N1 bird flu virus are similar to those of the common flu, including fever, cough, sore throat and muscle aches. The patient also can exhibit symptoms such as eye infection, pneumonia and severe respiratory illness.
