188 A. LWOFF 



had identified smallpox and knew that it was transmissible. Aristotle was 

 aware of the fact that rabies was transmitted by the bite of dogs; the Hebrews 

 used to compare this bite to that of a venomous snake. In Latin, virus means 

 "venom" or similar poisonous fluid. Virus was something which could 

 produce a disease. And in a.d. 50, Cornelius Aulus Celsus produced this 

 remarkable sentence: "Eabies is caused by virus." 



Ideas concerning infectious diseases remained metaphysical mitil the notion 

 of a specific agent emerged, and until, mainly as a consequence of Pasteur's 

 work, the agents of infectious diseases were identified as microbes. These 

 agents, whether bacteria, protozoa, or fungi, were called viruses. 



Pasteur et ah (1884) had no difficulty in proving that rabies was a specific 

 infectious disease. Although they were unable to see the agent, they quite 

 naturally considered it a small microbe. When Iwanowsky (1892) discovered 

 that the juice of tobacco plants showing the sjnnptoms of mosaic remained 

 infectious after filtration, he also concluded that the infectious agent was a 

 small microbe. Then came Beijerinck, who confirmed the filterability of 

 tobacco mosaic Virus. He also discovered that the infectious power was not 

 lost by precipitation with alcohol and that the infectious agents could diffuse 

 through agar gels. The infection, wrote Beijerinck, is not caused by microbes 

 but by a fluid infectious principle. This intuition of a difference of nature 

 between tobacco mosaic virus and microorganisms makes Beijerinck the real 

 founder of conceptual virology, that is to say, of virology. 



Beijerinck's views were so opposed to the current ideas that they did not 

 receive any attention. This did not prevent the discovery of a number of 

 filterable agents which were considered to be small infectious agents, in other 

 words, small microbes and which were therefore called filterable viruses or 

 ultra-viruses. Then someone remarked that, because ultraviruses were small 

 microbes, they should be called inframicrobes. For obvious reasons, none of 

 the scientists studying filterable infectious agents was pretentious or modest 

 enough to describe himself as an ultravirologist or as an inframicrobiologist. 

 And as everybody has to be labelled, these scientists were labeled as virolo- 

 gists. Quite naturally, the ultraviruses, the filterable invisible infectious 

 agents studied by virologists, became viruses. And, as a counterstroke, the 

 microbes, the visible infectious agents, were deprived of their ancestral 

 right to be called viruses. 



Today, most scientists agree to subdivide the infectious agents into two 

 groups: microbes — including bacteria, algae, protozoa, fungi — and viruses. 

 What is a virus? In order to answer this question, we shall consider bacterio- 

 phage as the model of a typical virus, and compare it with typical micro- 

 organisms and with typical cellular organelles. We will have to find out what 

 the discriminative characters are which allow us to separate viruses from 

 microorganisms and from organelles. This will lead us to discuss the notion of 



