Human influenza is a viral infection that typically recurs on an annual basis, usually during winter, in different mutations. These mutations are why an individual who recovers from the illness in one year might get influenza again the the next year. There are potentially an infinite number of kinds of human influenza, because influenza viruses are capable of infinite mutation and at any one time, there will be a mix of different viruses causing influenza. Some well-known mutations include the A, B and C types, the H1N1 subtype, the swine flu and the avian flu.
Influenza viruses are members of the orthomyxovirus family, and human influenza is caused by the A, B or C genus types. Each mutation or strain is given a name such as the H1N1 virus, a mutation of which caused a pandemic that spread around the world and killed more than 15,000 people in 2009. H1N1 is a subtype of the A virus, but it also has different strains or mutations, some of which cause a small number of human influenza outbreaks in any year and others that cause influenza in pigs, which is referred to as the swine flu, or in birds, which is called the avian flu.
Influenza typically spreads through airborne droplets, making them among the most infectious of human illnesses. Symptoms include a temperature of at least 100.4 degrees Fahrenheit (38 degrees Celsius), a sore throat, weakness and fatigue, aches and perhaps vomiting and diarrhea. The effects can be fatal in certain individuals through the development of viral pneumonia.
Sometimes adults who have had many years of exposure to different types of influenza or flu might have an immune system that can more quickly develop some resistance when a new virus mutation or strain emerges. This was found to be true of the H1N1 strain in 2009. Although they are more likely to have an immunity to a certain strain of human influenza, elderly people usually are a high-risk group during influenza outbreaks because they are not as healthy or strong as younger adults.
