Influenza is a respiratory infection caused by types A, B, and C flu virus, labeled by their protein makeup. The seasonal flu is a yearly infection with 36,000 fatalities in the United States, according to the National Institute of Allergies and Infectious Diseases. A pandemic flu, such as the swine (H1N1) flu, can spread worldwide without regard to season. The Spanish Flu of 1918-19 killed more than 20 million people and was an H1N1-type of flu.
A Sphere with Protein Quills
The flu virus is spherical in shape, with protein quills sticking out of the surface. The quills or spike-like proteins are of two types: hemagglutinin, or "HA," and neuraminidase, or "NA." The HA protein can stick to another cell to start the infection while the NA gives the virus an "exit strategy" so new formed viruses can leave the host and migrate to others, according to the NIAID.
A, B, Cs of Flu Viruses
Type A flu viruses can mutate and find hosts among ducks, chicken, pigs, humans and even whales, according to the NIAID. Type B flu migrates among human hosts. Type C finds hosts in humans, dogs and pigs but tends to be milder than A type flu viruses and doesn't start epidemics.
Typical Symptoms
Typical symptoms of the seasonal flu are sore throat and cough, muscle aches, runny nose and headaches with a fever that may be high. The NIAID says the infection is mainly respiratory in nature but can cause nausea and vomiting, especially in children. The H1N1 flu virus has been reported to cause vomiting and diarrhea in some people
Flu Complications
Flu viruses weaken the immune system of the host and allow complications to set in. A person who has the flu may well contract bacterial pneumonia or develop ear or sinus infections. According to the NIAID a patient could also suffer dehydration, and any underlying chronic conditions, such as heart disease, asthma and diabetes, could be worsened to a fatality.
Flu Infection
Flu infection spreads among people by way of mucous droplets in coughing, sneezing or from touching an infected person and bringing the infection from hand to mouth or nose. Because infection can occur a day before the patient feels sick and a week thereafter, a person can infect others before knowing of the illness and during the illness, according to the NIAID.
