Taxes, national service, diplomacy. A far cry from immunology and virology. Unless you are a blogger or interested in words. Then you get to bring them together.
First, taxes and national service. In Rome of old, these were things you owed to the Republic (as opposed to the Rome of today, the things you are expected to evade). The Latin word was munis, “sense of duty” (cf., “municipal”). Not everyone has a sense of duty (you noticed?). Even today — imagine — some people don’t have to serve (they are given foreign policy roles in Republican administrations) or are exempt from paying their share of taxes (these people are called major campaign donors). Ancient Rome had these people, too (NB: the Empire didn’t last). They were called immunis, exempt from service. The old use is still visible in “diplomatic immunity.”
When the word insinuated itself into English, around 1400, it generalized a bit to mean protection from bad stuff, but according to Katherine Barber’s book, Six words you never knew had something to do with pigs (highly recommended), it was used infrequently. According to Barber (who is Editor in Chief of the dictionary department of Oxford University Press in Canada), it was in reference to Louis Pasteur’s 1881 use of an attenuated vaccine to prevent anthrax in sheep that it was first used in the medical sense (I read Pasteur’s paper, just linked, and the word doesn’t appear there, but Barber is careful to say “in reference to,” although she doesn’t give a cite). She goes on to say the English word “immunize” was used to translate a German text in 1892.
One word that is in the Pasteur paper is “virus”:
In summary, we now possess a vaccine of anthrax which is capable of saving animals from this fatal disease; a virus vaccine that is itself never lethal; a live vaccine, one that can be cultivated at will and transported without alteration. (Originally published in Comptes Rendus de l’Academie des Science 92:1378-1383, June 13, 1881. Translated by Tina Dasgupta, Yale School of Medicine, Original Contributions Editor,Yale Journal Of Biology And Medicine 75 (2002), pp. 59-62)
[From the translator's prefatory note: This paper, translated from the original report in French, describes the set-up and outcome of this public test of anthrax vaccination. Pasteur uses the term virus in its nineteenth century context meaning any agent that transmits disease rather than in its more restricted modern sense. It is interesting to note that neither here nor elsewhere does Pasteur provide a description of the methods of preparation of his vaccine or other experimental details. He kept these confidential.]
Barber notes that the word “virus” in Latin meant “a slimy liquid,” “poison” or “an offensive odor or taste.” When first used in English, around 1600 it referred to snake venom, later referring to harmful substances produced in the course of disease. The same root gives us “virulent.” Again it was Pasteur, proponent of the idea that microscopic living agents were the cause of diseases like anthrax and rabies, who noticed that the spinal tissues of rabid animals could transmit the disease but had no visible microbes within it. He speculated the causative agent was too small to see (he was right). Shortly afterward a similar observation was made by a Russian, Ivanovski, who noticed that a mash from the leaves of tobacco plants with mosaic disease could transmit the disease. However filtering the mash did not eliminate whatever was transmitting the disease, which became known as a “filterable virus.” (Since nobody paid attention to Russian science in those days, it was discovered independently by the Dutch scientist Beijerinck). It wasn’t until 1916 that Allard made a filter fine enough to stop whatever it was (what we now call Tobacco Mosaic Virus) causing this plant disease.
At the time of the 1918 influenza pandemic the word virus generally meant a microbe (presumed to be a bacterium) too small to be seen and able to pass through any but the finest sterilizing filters but not culturable with any known culture media:
The influenza epidemic afforded the opportunity to research the etiological agent and develop the idea of the virus. Experiments by Nicolle and Le Bailly in Paris were the earliest suggestions that influenza was caused by a “filter-passing virus,” (BMJ, 11/2/1918). They filtered out the bacteria from bronchial expectoration of an influenza patient and injected the filtrate into the eyes and nose of two monkeys. The monkeys developed a fever and a marked depression. The filtration was later administered to a volunteer subcutaneously who developed typical signs of influenza. They reasoned that the inoculated person developed influenza from the filtrate since no one else in their quarters developed influenza (JAMA, 12/28/1918). These scientists followed Koch’s postulates as they isolated the causal agent from patients with the illness and used it to reproduce the same illness in animals. Through these studies, the scientists proved that influenza was due to a submicroscopic infectious agent and not a bacteria, refuting the claims of Pfeiffer and advancing virology. They were on their way to discerning the virus and characterizing the orthomyxo viruses that lead to the disease of influenza. (Human Virology, Stanford)
Our modern notion of a self-replicating obligate intracellular parasite with a characteristic life-cylce came much later.
Maybe more than you wanted to know. But I enjoyed it.