It’s hard to imagine a world in which we don’t hear anything about viruses, but we don’t have to travel too far back in time to see it be a reality. Scientists have discovered viruses as old as 30,000 years, yet our knowledge of their existence is much younger than that. So, how did we discover viruses?

The discovery of viruses is the result of a number of interesting events – and the story begins in 1796. When Edward Jenner developed the process of vaccination against smallpox, and with it produced the first ever vaccine for any disease, he did so without knowing that smallpox was caused by a virus, or what a virus was altogether. But, Jenner’s method of developing a vaccine kickstarted the development of other vaccines for more diseases.

Louis Pasteur, one of history’s greatest microbiologists and medical scientists, who developed a vaccine for rabies, came a step closer to the discovery of viruses than Jenner when he proposed that the pathogen responsible for rabies was much smaller in size than other known pathogens, and was too small to be detected by microscopy.

It was not until 1884, when Charles Chamberland and Louis Pasteur developed the Chamberland-Pasteur filter that the small mystery pathogens could be isolated from other cells and bacteria in a sample. This porcelain filter was used by Dmitri Ivanovsky in 1892 to show that infectious extracts from tobacco plants with mosaic disease remained infectious after filtration, but concluded that the plant disease was a result of a water-soluble toxin.

Six years later, Martinus Beijernick carried out very similar experiments with filtered extras from diseased tobacco plants, but reported that the pathogenic agent responsible was a “contagious living fluid,” believing that the pathogen was liquid in form. These pathogenic fluids became known as filterable agents, and it was believed that they contained no solid particles. The term virus was later used to describe the filterable agents.

That same year, in 1898, Paul Frosch and Friedrich Loeffler showed that a fluid they obtained from calves suffering from foot and mouth disease proved to be infectious after filtration. This was the first piece of evidence of viruses infecting animals. Loeffler and Frosch concluded that the infectious agent was a small particle and not a fluid, and showed that heating the extract conferred immunity to cows and sheep against foot and mouth disease. This was the first ever vaccine containing inactivated virus.

Also in 1898, Giuseppe Sanarelli described the myxoma virus (a relative of the smallpox virus which infect rabbits) as a virus. He was the first to identify a virus without the use of filtration, but rather used centrifugation as a method to sterilise his samples.

In 1915, Frederick Twort, a British bacteriologist discovered a small agent which had infected and caused the death of his bacterial cell culture. Independently to Twort, two years later Félix d’Hérelle discovered what a microbe which had caused the death of his bacterial culture; he called the virus a bacteriophage.

More and more viruses were discovered throughout the twentieth century, and new ones still continue to be discovered; it is estimated that there are 1x10³¹ viruses on earth, and studies suggest that there are at least 320,000 different species of viruses that infect mammals. Although we are far from knowing everything about the inner workings of viruses, we’ve made so much progress from the first discovery of viruses in the late nineteenth century.