As microscopes improved in resolution and acuity, they ushered in a race to identify microbes that caused human disease. Some pathologists began to suspect that there were infective agents even smaller than bacteria that could not be seen under an ordinary microscope. These tiny organisms could neither be isolated nor cultivated, although it was clear that the fluid in which they lived could infect another.

In 1857, an agricultural student discovered an unknown disease on a tobacco farm and reported this to Adolf Mayer, director of the Wageningen Agricultural experiment station in the centre of Holland. Mayer studied it for several years and ruled out the possibility of it being a nutritional disease.

Not able to find a bacterium, fungus or nematode in the plants, Mayer took an extract from the stems and leaves of diseased plants, mixed it with water and found that when spread over healthy tobacco plants, it caused an infection. When passed through a filter paper, the fluid remained infective, but when passed through a double filter paper, it no longer caused the disease.

Heating the infective fluid to 80 °C for an hour also killed its infectiveness. He concluded it was very likely a disease caused by bacteria that were far too tiny to be visible through any means then known.

The disease and its agent remained unclassified until 1890 when Dmitri Ivanovsky of St Petersburg University was sent out by the Department of Agriculture to study a disease afflicting tobacco plantations in Ukraine and Crimea. Like Mayer before him, Ivanovsky found the sap to be infective.

He passed diluted sap through a specialised filtering device called “Chamberland filter candles” and it remained potent to infect healthy plants. At first, he thought that the filters were defective, but eventually, like Mayer, Ivanovsky, too, concluded that the infective agent was a very tiny bacterium. It is Ivanovsky who is credited for discovering the first virus, not Mayer.

The term “virus” was actually coined by a third scientist who was studying the cause of mottling in tobacco leaves. Martinus Willem Beijerinck, son of a tobacco dealer who went bankrupt, did his doctoral dissertation on plant “galls” at the University of Leiden in 1877. He began teaching at the Agricultural School in Wageningen where he met Mayer, and they exchanged ideas on Mayer’s early observation and experiments in 1885 on the transmission of tobacco mosaic disease.

Like Mayer, Beijerinck, too, was unable to narrow down the data and discover the pathogen and he abandoned this pursuit and turned his attention to other things. However, in 1895, Beijerinck joined Delft Polytechnic as professor and resumed his study on diseases of the tobacco leaf. Just as Ivanovsky had done before him, Beijerinck reasoned that the cause of tobacco mosaic disease might be a bacterial toxin (similar to the one produced by tetanus bacilli, specifically an exotoxin [exo: outside]) where the bacteria, although present in the body, is not located at the site of the symptoms.

When Beijerinck experimented using a very small amount of it in both wet and dry form, he discovered it could infect numerous tobacco leaves. Exotoxins are proteins and easily rupture when dried or heated, so he concluded that this could not be of bacterial origin. Beijerinck’s tobacco extract survived in cold winters and warm summers in the soil, and on dried sun-cured tobacco leaves, and remained infective.

Despite repeated efforts, Beijerinck could not find and identify the causative agent and he concluded that the disease was not caused by bacteria but by a contagium vivum fluidum (Latin; living fluid infection), a clunky term that he dropped when he adopted the archaic Latin term “virus”.

To the etymologically inclined, the word “virus” is in some sense the opposite of itself. The root of the word derives from Sanskrit (विष, vish) and in its Latinised meaning, denotes venom or poison. In Middle English, the use of the word got expanded and “virus” came to mean semen. The word, therefore, signifies both birth and death at once. In a modern context too, “going viral” suggests that a message or image quickly reaches untold numbers of people, much in the manner in which an outbreak transmogrifies into a pandemic.

Some time before the word “virus” was coined, a major discovery had taken place in France. On 4 July 1885, a nine-year-old boy named Joseph Meister from Meissengott in the region of Alsace was savagely bitten by a rabid dog fourteen times in all, on his hand, legs and thighs. The local physician had heard about Louis Pasteur’s promising work with rabies in dogs and urged the Meisters to take their son quickly to Paris.

Mrs Meister and Joseph reached Pasteur’s clinic in Paris in the afternoon of 6 July. Pasteur consulted two leading physicians and friends, Alfred Vulpian and Jacques-Joseph Grancher, and considered vaccinating the boy with an early vaccine he had been testing on dogs, for which the results had been promising. Meister’s mother was uncertain but Grancher convinced her that the boy was doomed unless Pasteur tried the untested vaccine, and Meister’s mother begged Pasteur to save her son.

A close collaborator, friend, a famous bacteriologist in his own right and co-discoverer of the cure for diphtheria with Pasteur, Emile Roux chose not to participate in this trial, fearing adverse consequences. He was censorious of the haste with which Pasteur, Grancher and Vulpian were proceeding. He believed that all three of them were disregarding more humane considerations in their unswerving pursuit of scientific knowledge, and perhaps, commercial gain, for the institute.

Pasteur had been working on a vaccine to protect dogs against rabies and had shown that it worked successfully in protecting fifty dogs on whom he had tried his vaccine. But the results had not yet reached the larger scientific community or the public. Pasteur’s initial aim was to inoculate dogs and not humans with the vaccine until Meister and his mother arrived unexpectedly at his clinic.

Pasteur knew that, like polio, rabies infects the spinal cord. He had taken spinal cords from people who had died from the bites of rabid dogs and had serially passaged the rabies “agent” into dogs and then into rabbits. He had extracted a rabbit’s spinal cord and dried it in a flask to protect it from moisture and bacterial contamination. He then crushed the dried spinal cord into a powder and reinfected the spine of a fresh rabbit, repeating the process several times until the symptoms of the disease had become completely benign.

Vaccine designers call this process “attenuation”, a gradual reduction in the severity of diseases in a living virus. Today’s vaccines for measles, mumps and rubella (MMR) are all examples of similar live, “attenuated” virus vaccines.

Pasteur’s “vaccine” saved Meister’s life. Remember, this was a time before viruses had been isolated or even conclusively identified. What this means is that effective vaccines against a viral disease like rabies were made even before the agent causing the disease had been found!

Although Pasteur had found a cure for rabies, he had not looked too closely for the pathogen or, for that matter, even checked if the rabies-causing pathogen could pass through Chamberland’s fine filters. Ironically, the Chamberland filter, also known as the Pasteur-Chamberland filter, a foot-long metal tube packed with crushed porcelain, was developed by Pasteur’s assistant, Charles Chamberland, in 1884.

Had Pasteur or Chamberland run the rabies-causing pathogen through their porcelain filter, one of them would almost certainly have isolated the rabies agent and become discoverers of the pathogen ahead of Ivanovsky, or would have had the pleasure of giving the agent a name before Beijerinck did.

In 1903, Ivanovsky published a paper describing “abnormal crystal intracellular inclusions” in the host cells of affected tobacco plants that he proved were capable of reinfection. By this time, even though they still could not be seen, more and more new viral agents were being isolated. In 1898, the same year Beijerinck’s work was published, foot-and-mouth disease in cattle became the first animal illness linked to a filterable agent.

In 1901, American army doctors studying yellow fever in Cuba also found a mosquito-vectored pathogen that was filterable. In 1935, American biochemist Wendell Meredith Stanley was able to crystallise the tobacco mosaic virus (TMV) and show that it remained infective. This property helped in isolating and concentrating the virus.

Excerpted with permission from Invisible Empire: The Natural History of Viruses, Pranay Lal, Penguin Books India.