What is the flu (influenza)?
Influenza, commonly known as ‘the flu’, is an infectious disease caused by a group of influenza viruses. In healthy individuals, influenza is typically self-limiting, mild(ish), and rarely fatal. But it can be deadly in high-risk groups.
How common is flu?
It is very common, and very contagious. In a typical year, 5-15% of the population contract influenza. There are 3-5 million severe cases annually; and 400,000 deaths (which is 2% of all respiratory death). Deaths most commonly occur in high-risk groups, including young children, the elderly, and people with chronic health conditions. In temperate regions of the world, the number of influenza cases peaks during winter, whereas in the tropics influenza can occur year-round.
What are the symptoms of flu?
Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms begin from one to four days after exposure to the virus (typically two days) and last for about 2-8 days.
Diarrhoea and vomiting can occur, particularly in children. Influenza may progress to pneumonia, which can be caused by the virus or by a subsequent bacterial infection. Other complications of infection include acute respiratory distress syndrome, meningitis, encephalitis, and worsening of pre-existing health problems.
It is infectious one day before symptoms start, and most contagious in the first 3-4 days after the illness begins.
What are influenza viruses?
There are four types of influenza virus, termed influenza viruses A, B, C, and D:
- Influenza A virus (IAV) – aquatic birds are the primary source of Influenza A virus (IAV), which is also widespread in various mammals, including humans and pigs
- Influenza B virus (IBV) and Influenza C virus (ICV) – primarily infect humans
- Influenza D virus (IDV) – is found in cattle and pigs.
IAV and IBV circulate in humans and cause seasonal epidemics, and ICV causes a mild infection, primarily in children. IDV can infect humans but is not known to cause illness. In humans, influenza viruses are primarily transmitted through respiratory droplets produced from coughing and sneezing.
Diagnosis
Laboratory testing is not necessary in mild to moderate cases. But if it is needed, influenza infection is diagnosed with laboratory methods such as antibody or antigen tests and a polymerase chain reaction (PCR) to identify viral nucleic acid.
Treatment
The disease is normally self-limiting, i.e. its gets better of its own accord. So it can be treated with supportive measures. But, in severe cases, hospital admission and antiviral drugs such as oseltamivir, are required
Prevention and vaccination
Frequent hand washing and covering one’s mouth and nose when coughing and sneezing reduce transmission.
Influenza viruses, particularly IAV, evolve quickly, so flu vaccines are updated regularly to match the influenza strains in circulation. Vaccines currently in use provide protection against IAV subtypes H1N1 and H3N2 and one or two IBV subtypes.
Annual vaccination can help to provide protection against influenza.
Pandemics
Since the late 1800s, large outbreaks of novel (new) influenza strains that spread globally, called pandemics, have occurred every 10-50 years. Five flu pandemics have occurred since 1900: the Spanish flu in 1918–1920, which was the most severe flu pandemic, the Asian flu in 1957, the Hong Kong flu in 1968, the Russian flu in 1977, and the swine flu pandemic in 2009.
