Influenza is a term that covers a number of viruses. There are three main types of influenza, called simply Type A, Type B, and Type C. These viruses are spread in similar ways, passing through the air or traveling from person to person by the touching of surfaces that have been contaminated with the virus. There are also influenza viruses that typically infect animals, but which can mutate and spread to humans.
The type of flu that typically explodes into pandemic proportions is Type A. Within Type A, there are subtypes that are further broken down into strains. New strains within this type can appear, as in the case of the H1N1 virus that emerged in the spring of 2009 to cause a worldwide pandemic.
Not only humans suffer from Type A, but animals as well, and there are strains that pigs, birds, horses, and even whales are susceptible to. Some animals, such as pigs, ducks, and chickens, may host a number of different strains that can cross the boundaries between species and manifest themselves with similar symptoms. When animals are infected with two different strains at the same time, these can mutate to create a new virus, which can jump between species. This was the case with the H1N1 virus, popularly known as the swine flu, created from strains usually found in European and Asian pigs and birds that combined with human influenza. When this happens, it is known as an antigenic shift, and these strains of the flu are particularly debilitating because humans typically have little or no resistance to them.
Type B influenza has no subtypes, but does have a number of distinct strains. Symptoms are similar to those inflicted by Type A, but are generally milder. These symptoms include fever, chills, fatigue, headache, coughing, and runny nose. In children, these two types of influenza viruses can also include symptoms of nausea and vomiting.
The mildest form of influenza viruses is called Type C. Symptoms are similar to those of the common cold, and often involve mild respiratory distress. Though those afflicted with Type C do not display the flu symptoms associated with the others, the virus is spread in the same way as the other types of influenza.
Types A and B are the types of influenza typically targeted in vaccines. Vaccines do not protect against all strains, but can be effective against the strains that the immunization is designed for as well as those that are closely related. Individuals who receive the immunization are not protected against Type C viruses.
