Influenza is an acute viral respiratory infectious disease caused by influenza virus. Influenza viruses are divided into types A, B, C, and D, and each type of influenza virus has different transmission, symptoms, and epidemic characteristics. The following is a brief introduction to each type of influenza virus.
Influenza A: The 'protagonist' of influenza
The influenza A virus is the most common type of influenza and poses a significant threat to humans. The subtypes of influenza A virus are determined by two surface proteins: hemagglutinin (HA) and neuraminidase (NA). Currently, 18 types of HA and 11 types of NA have been discovered, with different combinations forming different subtypes of the virus.
The H1N1 and H3N2 subtypes of influenza viruses can cause seasonal outbreaks in the population, hence they are also known as "seasonal influenza". The peak of seasonal influenza outbreaks in China usually occurs in winter and spring, while small-scale influenza peaks also occur in the summer in southern regions.
The influenza A virus not only infects humans, but also various animals such as poultry, wild birds, pigs, horses, marine mammals, etc. Therefore, it is divided into avian influenza, swine influenza, equine influenza, etc. Common subtypes of avian influenza include H5N1, H5N6, H7N9, H9N2, etc. Common subtypes of swine influenza include H1N1v, H1N2v, H3N2v, H3N3, etc. Generally speaking, animal influenza is less likely to infect humans, but under certain conditions, certain animal influenza viruses may cross species barriers and infect humans. Once these viruses mutate and can effectively spread between people, they may trigger a larger outbreak, even a flu pandemic.
Type B influenza: periodic seasonal outbreaks
Type B influenza is also a seasonal flu, usually experiencing a peak of outbreaks every few years. The symptoms of influenza B are similar to those of influenza A, but it has never caused a global pandemic in history. The influenza B virus is divided into two lineages, B/Victoria and B/Yamagata. It is worth noting that since March 2020, no B/Yamagata virus has been detected in the global monitoring system.
The host of influenza B is relatively single, and currently only influenza B virus has been found in humans and seals. A small number of animals, such as ferrets or mice, can be infected with influenza B under experimental conditions.
Influenza C: A milder infection
The C influenza virus can infect humans, but it usually only causes mild respiratory infections with inconspicuous symptoms, mostly in children, and rarely triggers large-scale epidemics. The C influenza virus can not only infect humans, but has also been found in animals such as pigs, dogs, and cows. However, overall, influenza C poses less of a serious threat to human health.
Type D influenza: a rare animal infection
Type D influenza is even rarer, and no cases of human infection have been found so far. The D-type influenza virus mainly infects a few animals such as pigs and cows, and its impact range is limited.
There is no subtype classification for influenza C virus and influenza D virus.
