Influenza, commonly known as the flu, is a type of respiratory infection that can infect both animals and humans. According to the National Institute of Allergy and Infectious Diseases, flu viruses are classified according to their protein composition. Some flu types can infect only animals while others infect only people. Some types can infect both.
Seasonal Influenza
The seasonal flu, frequently referred to simply as the flu, is a type B influenza virus. Unlike type A flu, which infects a broad range of animals, type B is only known to affect humans and seals. The seasonal flu virus affects approximately 5 to 20 percent of the population of the United States every year, as reported by the CDC. Symptoms include a fever that may or may not be accompanied by chills, fatigue, cough, sore throat, muscle aches and a runny or congested nose. The seasonal flu occurs most often during the cold winter months. Vaccinations are available to help prevent transmission of the seasonal flu; however, it must be given every year for continued protection.
H1N1 or Swine Flu
The H1N1 flu was declared a pandemic in 2009, meaning the virus was widespread all over the world. Unlike the seasonal flu, H1N1 is easily transmitted during warm spring and summer months and has the ability to cause death in otherwise healthy individuals. The World Health Organization explains that symptoms of H1N1 include fever, coughing, joint or muscle pain, vomiting, diarrhea, headache, runny nose and sore throat. The National Institute of Allergy and Infectious Diseases notes that pandemic flu viruses are classified as type A, and are the most feared in the medical community. A vaccine is available against the H1N1 flu.
Bird or Avian Flu
The avian flu is an example of a type of flu that occurs in other species but isn't normally transmitted to humans. The Mayo Clinic explains that when the bird or avian flu is acquired by humans, over 50 percent of infected patients die. Commonly, the bird flu is passed from infected birds to people in close contact with them, but the virus has been transmitted between humans as well. Symptoms of the avian flu are the same as with the seasonal flu virus. Coughing, muscle aches, fever and sore throat are indicators of this flu, and occasionally an eye infection may occur. As of July 2010, there was no vaccine available to protect against the avian flu.
Type C Influenza
Type C flu viruses are the mildest of all the types. Individuals infected with a type C flu virus typically have very mild respiratory symptoms such as coughing and a runny nose, sometimes no worse than the common cold. This type of flu has never been associated with an epidemic, explains FluFACTS.com.
