The Influenza (Flu) Virus

ext to the common cold, influenza or "the flu" is perhaps the most familiar respiratory infection in the world. In the United States alone, approximately 25 to 50 million people contract influenza each year. The symptoms of the flu are similar to those of the common cold, but tend to be more severe. Fever, headache, fatigue, muscle weakness and pain, sore throat, dry cough, and a runny or stuffy nose are common and may develop rapidly. Gastrointestinal symptoms associated with influenza are sometimes experienced by children, but for most adults, illnesses that manifest in diarrhea, nausea, and vomiting are not caused by the influenza virus though they are often inaccurately referred to as the "stomach flu." A number of complications, such as the onset of bronchitis and pneumonia, can also occur in association with influenza and are especially common among the elderly, young children, and anyone with a suppressed immune system.

Influenza is highly contagious and is more common during the colder months of the year. Contrary to traditional belief, however, the climate itself is not directly to blame for the increase in incidence, but rather is attributable to the greater amount of time spent indoors in close proximity to other individuals during inclement weather. The influenza virus is chiefly transmitted through airborne respiratory secretions released when an infected individual coughs or sneezes. Incubation typically is from one to two days from the time of infection, and most people begin to naturally recover from symptoms within a week. The vast majority of influenza-related deaths are caused by complications of the flu rather than the actual influenza virus.

Three distinct types of influenza virus, dubbed A, B, and C, have been identified. Together these viruses, which are antigenically distinct from one another, comprise their own viral family, Orthomyxoviridae. Most cases of the flu, especially those that occur in epidemics or pandemics, are caused by the influenza A virus, which can affect a variety of animal species, but the B virus, which normally is only found in humans, is responsible for many localized outbreaks. The influenza C virus is morphologically and genetically different than the other two viruses and is generally nonsymptomatic, so is of little medical concern.

Posted in: Microbiology