Environmental Stability of the Virus
The influenza pathogen exhibits a moderate degree of environmental resistance. Its survival depends directly on temperature conditions. At low temperatures (around 4 °C), the virus can remain viable for up to one week. However, elevated temperatures are detrimental to the pathogen: it is rapidly inactivated when heated above 60 °C.
In addition, viral particles are extremely sensitive to ultraviolet radiation. An important structural feature is the presence of a lipid envelope, making the pathogen easily inactivated by lipid solvents and standard disinfectants. This makes regular wet cleaning and ventilation effective measures for reducing viral concentration indoors.
Transmission Mechanisms and Susceptibility
Influenza is a classic anthroponosis, meaning the main source of infection is a sick person. The infection is transmitted via two main routes:
- Aerogenous (primary): the virus is released into the environment during coughing, sneezing, or even simple conversation as a fine aerosol.
- Contact (secondary): infection occurs when viral particles are transferred via contaminated hands or fomites to the nasal mucosa and conjunctiva.
The contagiousness level is exceptionally high. Outbreaks are most frequently registered in closed communities, where ideal conditions exist for rapid pathogen exchange. The scale of epidemic spread depends directly on herd immunity, formed by previously infected and vaccinated individuals.
Susceptibility varies by age. Children fall ill significantly more often and experience more severe disease due to the lack of established, durable immunity. Meanwhile, adult patients, especially the elderly and those with underlying comorbidities, form the high-risk group with the highest mortality rates.
Role of Different Virus Types
The epidemiological picture is shaped primarily by two virus types: A and B.
Influenza A virus is of paramount epidemiological importance. It is unique in that it can infect not only humans but also various animals, including birds. This specific type is responsible for global pandemics with high mortality rates. Between pandemics (interpandemic intervals), influenza A causes annual seasonal epidemics.
Influenza B virus circulates exclusively among humans. It causes epidemic surges much less frequently—on average once every 4 to 6 years—and does not lead to pandemics.
Historical Pandemics and Current Situation
In the 20th century, humanity faced three devastating pandemics, each triggered by mutations of the influenza A virus:
- 1918–1920 — "Spanish Flu" (subtype H1N1): the most severe pandemic in history, claiming the lives of over 20 million people.
- 1957–1959 — "Asian Flu" (subtype H2N2): originated in Singapore, with the number of cases reaching 1.5–2 billion.
- 1968–1970 — "Hong Kong Flu" (subtype H3N2): affected approximately 1 billion people.
Currently, there is a simultaneous circulation of influenza A viruses (current subtypes H3N2 and H1N1) and influenza B. Vaccines are formulated annually to include these circulating variants for prophylaxis.
Despite active vaccination, influenza remains an uncontrolled infection. A key role in disease containment is played by the World Health Organization (WHO) global surveillance program.