Who Discovered Viruses and How did the smallest living organism complete Pasteur’s germ theory?

Far smaller than cells and bacteria, viruses are the smallest life forms on Earth, so small they can only reproduce inside some host cell and do it by taking over control of that cell. Viruses are so small they easily slip through virtually any filter or trap. Their discovery answered many medical questions at the beginning of the twentieth century and completed Pasteur’s germ theory.

Viruses cause many of the most dangerous human diseases. Until they were discovered, medical science had ground to a halt in its advance on curing these human illnesses. When Beijerinick discovered viruses, he actually discovered a new life form, one too small to be seen with any microscope other than a mighty electron microscope.

French scientist Louis Pasteur discovered germs (microscopic bacteria) and claimed that germs caused disease and rot. However, he was never able to find a microorganism (germs) that caused rabies, though he tried for over a decade before giving up in 1885. It left a shadow of doubt over his germ theory.

Another disease for which no one could find an identifiable causative agent was tobacco mosaic disease (so called because a mosaic pattern forms on the leaves of infected plants). In 1892 Russian botanist Dmitri Ivanovsky decided to search for this mysterious agent. (It was safer to work with tobacco mosaic disease than with deadly rabies.) Ivanovsky mashed up infected leaves and passed the fluid through various paper and ceramic filters. These filters were supposed to trap all organisms, even the tiniest bacteria.

However, the fluid that strained through these sets of filters could still infect healthy tobacco plants with mosaic disease. That meant that Ivanovsky hadn’t trapped the causative agent in his filters. He tried different filter materials, different treatments, and baths for the leaves and mashed juice. His results were always the same. Whatever caused this disease, Ivanovsky couldn’t trap it in a filter.

Ivanovsky refused to believe that any living organism existed that was smaller than bacteria and so concluded that his filters were defective and would not, in fact, catch small bacteria. In disgust, he abandoned his project.

In 1898 Dutch botanist Martinus Beijerinick decided to try his luck at solving the mystery of tobacco mosaic disease. He repeated Ivanovsky’s experiment and got the same result. However, Beijerinick was quite willing to conclude that this experiment proved that the causative agent was something new and unknown, something much smaller than bacteria. That was why it hadn’t been trapped in his filters. Beijerinick admitted that he did not know what it was, but he claimed that his experiment proved that it existed and that it was super-tiny. He named it a “virus,” the Latin word for poison.

While this discovery was intellectually interesting to some scientists, few cared about a disease unique to tobacco plants. The notion of viruses received little attention from the medical and scientific communities.

In 1899 German scientist Friedrich Loeffler conducted a similar test and concluded that the agent responsible for foot-and-mouth disease was too tiny to be bacteria and so must be another virus. Two years later, in 1901, American army surgeon Walter Reed exhausted his attempts to discover the cause for yellow fever that had killed so many American soldiers. Then he tested this mosquito-borne disease to see if whatever caused it was small enough to be a virus. It was.

This discovery convinced the scientific world that viruses, 1/1000 the size of even a small bacterium, were the cause of many human ailments and had to be studied and treated separately from bacteria. Ivanovsky and Beijerinick discovered viruses, but it took Walter Reed to make the medical and scientific community pay attention.

What’s the most common disease-causing virus? The common group of rhinoviruses, of which there are at least 180 types. Rhinoviruses cause colds and are almost universal, affecting everyone except for those living in the frozen wastes of Antarctica.