Causes

Influenza is caused by a virus. There are three types of viruses that cause influenza, Types A, B and C. Types A and B are the most severe and widespread.

• Influenza A Virus: This is the most common type of influenza virus, and has three subtypes commonly seen in the human population (H1N1, H2N1, H3N2). The influenza A Virus undergoes rapid minor changes that continually allow it to get past the body?s immune response system and cause infection; for this reason, new flu vaccines must be developed every year. This virus can infect birds, humans, dogs, horses and pigs. Epidemics caused by the A virus occur about every two to three years.
• Influenza B Virus: The influenza B Virus is the second most common type of virus responsible for influenza outbreaks. It is not classified into subtypes. The Influenza B virus undergoes minor mutations (changes) that cause people to remain susceptible to infection. This virus is known to infect humans and seals only. Epidemics caused by the B virus occur about every four to five years.
• Influenza C Virus: The Influenza C virus is less common than either A or B. It usually causes a mild upper respiratory tract infection with fever, cough and runny nose. The majority of humans develop antibodies to the C virus early in life. The influenza C virus is commonly found in respiratory infections in children less than six years of age. Epidemics caused by this virus occur only sporadically.

What type of virus causes the Avian Flu (Bird Flu)?

The Avian Flu (Bird Flu) is caused by the Influenza A Virus, subtype H5N1. It is highly contagious and transmitted through contact with bird droppings from infected birds and possibly contact with infected pigs. It does not appear to be readily passed between humans. The concern regarding this virus is the severity of the infection it can cause. Greater than 50% of humans infected with the H5N1 virus have died from the infection. It is feared that if the virus mutates into a form that can be readily transmitted between humans it could lead to a global pandemic.