Introduction
Flu is a word commonly used to describe conditions involving upper respiratory symptoms and some systemic complaints such as fever or headaches. It is derived from infections by the influenza viruses, which cause specific disease syndromes, though it has been used loosely for infections by several other pathogens affecting the respiratory system. There are three types of the influenza virus. Types A and B are usually involved in human disease while type C is much less often cited in outbreaks. The Centers for Disease Control and Prevention estimates that 36,000 people die yearly from the flu, 200,000 people are hospitalized and millions are infected.
Infection and Outbreaks of Influenza
The influenza virus usually spreads by inhaling droplets that were spread by sneezing, sniffling or coughing carriers. The virus has limited survival outside the body and can be spread by touching infected surfaces, with subsequent direct transfer to the nose. The outbreaks are usually seasonal, beginning as early as October and stretching into March of the next year. Infection is helped by the cold season, when the respiratory system is most challenged, with mucus layers thinned by the cold air. The flu virus produces an enzyme, neuraminidase, which further neutralizes the protective ingredients in mucus linings. Another contributing factor to outbreaks is the ever-changing nature of the flu virus. The flu virus is capable of changing key features on its surface, rendering it unrecognizable to the body defenses (called antigenic drift). This is what makes yearly flu vaccines necessary. Influenza A goes even further, completely overhauling itself to produce new strains (antigenic shift) that give rise to pandemics and a more severe disease to a totally unprepared immune system.
Influenza Virus in the Body
Once in the body, the virus binds to respiratory lining cells by means of special proteins on the viral envelop, which it usually changes from year to year. It is this binding that is blocked by the body's defenses following vaccination. After gaining entrance to the cell, the virus hijacks cellular processes to manufacture viral parts and assemble new viruses, which are then released from the cell, usually into the tubes of the respiratory tree.
The infection of the lining of the respiratory tree leads to wide spread cell death among these cells, with breakdown of their ability to wave the cilia they carry and clear secretions outward. Infected cells are also recognized as foreign by the body defenses, which release chemicals called interferons to kill these cells, further worsening the clearance of mucus secretions. Inflammatory agents are released that mediate the systemic symptoms of fever, weakness and body pains. Local inflammation accounts for the cough, croup and difficulty in breathing. Any other aspect of the respiratory system can be affected in severe disease.
Symptoms and Complications of Influenza
Symptoms of the flu include a high fever, dry or sore throat, cough, stuffy or runny nose and muscle pains. With increasing severity, influenza infection can lead to widespread destruction in the lungs, causing pneumonia. It can also open up enough gaps for other bacteria, particularly Staphylococcal species, to take root and cause pneumonia. The compromised respiratory condition caused by this situation is the usual cause of death. This scenario is made much worse in pre-existing challenges to the respiratory system such as asthma, heart disease and pregnancy.
Everyone is encouraged to get the flu vaccine to prevent a flu infection. Those at increased risk for severe disease are particularly encouraged to get vaccinated.
