502 SCOTTISH METROPOLITAN VETERINARY MEDICAL SOCIETY. 
the plague; and in 1730 Dr. Thomas Fuller, who has given us the first 
exact description we possess of sheep-pox in this country, is very 
explicit. He writes: “ My settled opinion is, that in regard every 
effect is necessarily such as its cause; it must needs be that every sort 
of venomous fever is produced by its proper and peculiar species of 
virus. And that the manner and symptoms of every such fever is not 
so much from the particular constitution of the sick as from the 
different nature and genus of their specific venom which caused them. 
And I conceive that venomous febrile matters differ, not in degree of 
intenseness only, but in essence and toto genere also; and that venemous 
fevers are for the most part contagious.” {Exanthematologia, or an Account 
of Eruptive Fevers, especially the Measles and Small-poo:: London, 1730.) 
Dr. Fuller also speaks of the contagia as particulate, or consisting of 
particles. 
But it must be confessed that the notions regarding contagious dis¬ 
eases were hazy in the extreme, and it was not until the experimental 
method and the microscope had been introduced into pathological re¬ 
search that our knowledge of them began to be exact and comprehen¬ 
sive. The idea that the active principles of the contagia were vital, and 
that each possessed individual characters, was until then mere speculation. 
I think the time has now arrived when the knowledge thus acquired 
should be utilised in formulating something like fixed principles with 
regard to these diseases, and deciding once and for all whether con¬ 
tagious disorders can or cannot be spontaneously generated. From 
almost every-day evidence it would seem that no definite opinions are 
held on this point; and in our own profession the preponderating 
tendency is, perhaps, to consider them as capable of being originated, 
at least in some instances, without any pre-existing contagium. This 
appears to be the case, at any rate, with two diseases—rabies and 
glanders. I have myself held this opinion, and am ready and willing to 
confess that, whenever I am confronted with convincing evidence of 
error, I am not ashamed to admit that I entertained erroneous ideas. 
We may be wise at twenty, but we should be wiser at forty. For 
certain other maladies of an exotic kind—as rinderpest, foot-and-mouth 
disease, sheep-pox, and zymotic pleuro-pneumonia—I have never ad¬ 
mitted their possible spontaneous development, at least in this country; 
but with regard to glanders and rabies, I have, until not long ago, 
remained in doubt. I am now entirely of opinion that no contagious 
disease can be so generated, and the reasons for arriving at this opinion 
I will give. 
By the term contagious or infectious, I do not refer to those disorders 
solely due to animal or vegetable parasites, of which scabies and tinea 
may be taken as examples; it is generally accepted, I believe, that the 
acari and epiphytes producing these diseases cannot be developed except 
from pre-existing organisms of the same kind. By the above terms I 
mean maladies which are transmitted from a sick animal to a healthy 
one by mutual touch or contact, or by particles of matter from the 
former, suspended in the atmosphere or carried by various media— 
particles which find admission into the system of the exposed animal 
by one channel or another, and constitute what is designated the “ con¬ 
tagium ” or “virus ” of these diseases. What differentiates this poison 
from other animal poisons, such as that of reptiles or septicaemia, is that 
the minutest quantity will, when introduced into the body, produce its 
effects as certainly as the largest, owing to its peculiar property of in¬ 
definite multiplication, and give rise to as marked effects in the 
thousandth generation as in the first. 
