Influenza

Influenza, often called the flu, is an infectious illness triggered by influenza viruses. Symptoms can be mild or severe and usually include fever, runny nose, sore throat, muscle aches, headache, cough, and fatigue. These signs appear one to four days after exposure—typically within two days—and last for about two to eight days. Children may also experience diarrhea and vomiting. The flu can lead to pneumonia, either from the virus itself or from a secondary bacterial infection. Other possible complications include acute respiratory distress syndrome, meningitis, encephalitis, and worsening of existing conditions like asthma and heart disease.

There are four types of influenza virus: A, B, C, and D. Influenza A virus mainly comes from aquatic birds but also infects many mammals, including humans and pigs. Influenza B and C viruses primarily infect humans, while influenza D is found in cattle and pigs. Types A and B circulate among people and cause seasonal epidemics; type C usually causes mild illness, mostly in children. Type D can infect humans but doesn’t appear to cause disease. The viruses spread mainly through respiratory droplets from coughs and sneezes, as well as through aerosols and contaminated surfaces.

Transmission can be reduced by frequent hand washing, covering the mouth and nose when coughing or sneezing, and wearing a mask. Annual vaccination helps protect against the flu. Because influenza viruses—especially type A—evolve quickly, vaccines are updated each year to match circulating strains. Vaccines target influenza A subtypes H1N1 and H3N2, plus one or two influenza B subtypes. Diagnosis is done with lab tests like antibody or antigen tests and PCR to detect viral genetic material. Treatment involves supportive care, and in severe cases, antiviral drugs like oseltamivir. For healthy people, the flu is usually self-limiting and rarely fatal, but it can be deadly for high-risk groups.

Each year, 5 to 15 percent of the population gets the flu. There are 3 to 5 million severe cases globally, leading to up to 650,000 respiratory-related deaths annually. Deaths mostly occur in high-risk groups: young children, the elderly, and people with chronic health conditions. In temperate regions, flu cases peak in winter; in the tropics, the virus circulates year-round. Since the late 1800s, new pandemic strains have emerged every 10 to 50 years. Five flu pandemics have occurred since 1900: the Spanish flu (1918–1920, the most severe), the Asian flu (1957), the Hong Kong flu (1968), the Russian flu (1977), and the swine flu pandemic (2009).

**Signs and symptoms** Flu symptoms resemble those of a cold but are usually more severe and less likely to include a runny nose. The incubation period is one to four days, most often one to two days. Many infections cause no symptoms. Onset is sudden, starting with non-specific signs like fever, chills, headache, muscle pain, malaise, loss of appetite, fatigue, and confusion. These are often followed by respiratory symptoms: dry cough, sore or dry throat, hoarseness, and a stuffy or runny nose. Coughing is the most common symptom. Gastrointestinal issues like nausea, vomiting, diarrhea, and gastroenteritis can occur, especially in children. Typical symptoms last two to eight days. Some studies suggest the flu can cause long-lasting symptoms, similar to long COVID.

Most symptomatic infections are mild and limited to the upper respiratory tract, but pneumonia is fairly common. Pneumonia can be viral or caused by a secondary bacterial infection. Primary pneumonia involves rapid fever, cough, labored breathing, and low oxygen levels leading to bluish skin. It is especially common in people with underlying heart conditions like rheumatic heart disease. Secondary pneumonia usually appears after a one- to three-week improvement period, with recurrent fever, sputum production, and fluid in the lungs, though it can also develop just days after flu symptoms start. About one-third of primary pneumonia cases are followed by secondary pneumonia, often caused by *Streptococcus pneumoniae* or *Staphylococcus aureus*.

**Virology** **Types of virus** There are four influenza virus species, each the sole member of its own genus within the family Orthomyxoviridae: Influenza A virus (genus *Alphainfluenzavirus*), Influenza B virus (*Betainfluenzavirus*), Influenza C virus (*Gammainfluenzavirus*), and Influenza D virus (*Deltainfluenzavirus*). Influenza A causes most severe illness, seasonal epidemics, and occasional pandemics. It infects all ages but disproportionately affects the elderly, the very young, and those with chronic conditions. Birds—especially aquatic ones like ducks, geese, shorebirds, and gulls—are the main reservoir, but the virus also circulates in mammals such as pigs, horses, and marine mammals. Subtypes of Influenza A are defined by the combination of two viral envelope proteins: haemagglutinin (H) and neuraminidase (N).