What is Influenza also called Flu? How Flu Spreads. Who is most likely to be infected with influenza? How is seasonal incidence of influenza estimated?
Table 1. Period of Contagiousness. People with flu are most contagious in the first days after their illness begins. Some otherwise healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick. Some people, especially young children and people with weakened immune systems, might be able to infect others for an even longer time. Onset of Symptoms. Complications of Flu. People at High Risk from Flu.
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In some cases, it can lead to death. Influenza A viruses cause seasonal flu epidemics practically every year in the United States. They can infect humans and animals. Influenza A is the only type that can cause a pandemic, which is a global spread of disease.
Bird flu and swine flu pandemics both resulted from influenza A viruses. An influenza A virus has two surface proteins: hemagglutinin and neuraminidase.
These help doctors with classification. Influenza B viruses can also cause seasonal epidemics that typically only affect humans. There are two lineages of influenza B: Victoria and Yamagata. Influenza B viruses mutate more slowly than influenza A viruses. Influenza C viruses cause mild illnesses — they do not appear to cause epidemics. Influenza D viruses mainly affect cattle and do not seem to infect humans.
Alternately, a person can catch the flu if they touch a surface contaminated with the flu virus, then touch their own mouth or nose. The CDC report that people with the flu are the most contagious in the 3—4 days after becoming ill.
Symptoms tend to develop 2 days after the illness starts, so a person may pass on the flu before they feel sick. For a person who is generally healthy, the flu is not typically dangerous. However, it can severely affect certain groups of people, who should seek medical attention as soon as they suspect that they have flu symptoms. Those most at risk of developing flu complications include:. Many people believe that influenza A is more severe than influenza B.
However, this is not always the case. A study concluded that adults hospitalized with influenza A or B tended to have similarly long hospital stays. An influenza clade or group is a further subdivision of influenza viruses beyond subtypes or lineages based on the similarity of their HA gene sequences.
See the Genome Sequencing and Genetic Characterization page for more information. Clades and subclades are shown on phylogenetic trees as groups of viruses that usually have similar genetic changes i. Dividing viruses into clades and subclades allows flu experts to track the proportion of viruses from different clades in circulation.
Note that clades and sub-clades that are genetically different from others are not necessarily antigenically different. These proteins act as antigens. Antigens are molecular structures on the surface of viruses that are recognized by the immune system and can trigger an immune response such as antibody production. Therefore, for antigenically different viruses, immunity developed against one of the viruses will not necessarily protect against the other virus as well.
Influenza A H3N2 viruses also change both genetically and antigenically. Influenza A H3N2 viruses have formed many separate, genetically different clades in recent years that continue to co-circulate.
Similar to influenza A viruses, influenza B viruses can then be further classified into specific clades and sub-clades. Influenza B viruses generally change more slowly in terms of their genetic and antigenic properties than influenza A viruses, especially influenza A H3N2 viruses.
Influenza surveillance data from recent years shows co-circulation of influenza B viruses from both lineages in the United States and around the world. However, the proportion of influenza B viruses from each lineage that circulate can vary by geographic location and by season. Figure 2 — This image shows how influenza viruses are named. The name starts with the virus type, followed by the place the virus was isolated, followed by the virus strain number often a sample identifier , the year isolated, and finally, the virus subtype.
CDC follows an internationally accepted naming convention for influenza viruses. This convention was accepted by WHO in and published in February in the Bulletin of the World Health Organization, 58 4 see A revision of the system of nomenclature for influenza viruses: a WHO Memorandum pdf icon[ KB, 7 pages]external icon pdf icon external icon. The approach uses the following components:. Getting a flu vaccine can protect against these viruses as well as additional flu viruses that are antigenically similar to the viruses used to make the vaccine.
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