Kolkata, Nov 17 : Influenza, commonly called “the flu”, is an illness caused by influenza viruses that infect humans. These viruses are transmissible between humans and are known as seasonal influenza viruses.
The most common symptoms of influenza include fever, a dry cough, headache, muscle and joint pain, a sore throat and a runny nose.
Each year, millions of people come down with flu. Most recover within a week, but for an unlucky few, flu can be deadly. Seasonal flu epidemics typically occur in late autumn and winter. Seasonal influenza is an acute respiratory infection caused by influenza viruses which circulate in all parts of the world.
In tropical regions, influenza can occur throughout the year, causing outbreaks more irregularly, but prevention is still important.
Flu is contagious. It spreads easily in crowded spaces, such as on public transport, in schools and nursing homes and during public events. When an infected person coughs or sneezes, droplets containing the virus can spread as far as one metre, and infect others who breathe them in.
There are 4 types of seasonal influenza viruses, types A, B, C and D. Influenza A and B viruses circulate and cause seasonal epidemics of disease. Influenza A viruses are further classified into subtypes according to the combinations of the hemagglutinin (HA) and the neuraminidase (NA), the proteins on the surface of the virus. Currently circulating in humans are subtype A(H1N1) and A(H3N2) influenza viruses. The A(H1N1) is also written as A(H1N1)pdm09 as it caused the pandemic in 2009 and subsequently replaced the seasonal influenza A(H1N1) virus which had circulated prior to 2009. Only influenza type A viruses are known to have caused pandemics.
Influenza B viruses are not classified into subtypes, but can be broken down into lineages. Currently circulating influenza type B viruses belong to either B/Yamagata or B/Victoria lineage.
Influenza C virus is detected less frequently and usually causes mild infections, thus does not present public health importance. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people. (UNI)