Viruses that cause influenza and other respiratory diseases
The influenza virus, a highly contagious disease with acute respiratory symptoms, has plagued mankind since ancient times. In the distant past, there are numerous descriptions of epidemics of severe respiratory illness with high fever in a geographical area. These epidemics usually lasted for a few weeks and then ended as quickly as they started. The first written record of this type of epidemic dates from 412 BC and was published by Hippocrates, one of the most famous physicians of antiquity.
In analysing respiratory epidemics, the following regularities have been discovered
While influenza has killed untold millions of people over the centuries, between November 1918 and February 1919 it caused an unprecedented pandemic. Recorded in the black pages of history as the “Spanish Flu”, it killed more than 20 million people in four months. Far more people died in the epidemic than in the entire First World War. This massive epidemic accelerated research to detect the pathogen.
In the late 1930s, it was discovered that chicken eggs hatched for 8-11 days were very suitable for breeding the influenza virus. With the discovery of tissue culture in the 1950s, the breeding of the influenza virus was simplified but also made much more efficient. And in the late 1950s, the electron microscope made it possible to visualise viruses to the human eye.
Frequency
Outbreaks occurred relatively frequently, but in completely irregular periods. Between two epidemics the disease disappeared completely.
Severity
The epidemics were not always equally severe, but it was very typical that mortality was much higher among older people than among younger people.
Distribution
Some epidemics, such as those of 1791 and 1830, originated in Asia and reached Europe via Russia.
The influenza viruses
The family Orthomyxoviridae consists of two genera, Influenza A and B and Influenza C. Pandemics (1889, 1918, 1957, 1968, 1977) are caused exclusively by subtype A, and epidemics by variants of type A and B. The influenza C virus causes only scattered cases, possibly domestic epidemics, with a milder course, mainly in children. The influenza A virus can affect many animal species other than humans (waterfowl, pigs and horses being the most epidemiologically relevant), the B virus is exclusively found in humans, while the C virus is mainly found in humans, but has also been isolated from pigs in China.
