As already said, later physicians thought that contagious diseases were propagated in three different ways, 1st, by actual contact with the matter or virus itself; 2dly, by fomites, or by contact with clothes tainted with it; and, 3dly, by infection, or by air holding it in solution.
But it is to contagion, as diffused through the air, that the observations that are to follow are directed. So we shall, for the present at least, dismiss the other two modes of its action, that by contact, and that by fomites or tainted clothes, with the expression of our belief in their existence, as modes of the propagation of disease.
In 1777, Dr Haygarth, an English physician, began to investigate the laws that regulate the action of contagious poisons, and for the first time they obtained a scientific examination, and became the subject of experiment, if, perhaps, are excepted the labours of Lind, whose observations appeared about the same time.
Dr Haygarth believed in the propagation of disease through the direct application of contagious matter, such, for instance, as that of small-pox; but of this none have expressed any doubt worthy of notice; for the fact is well known, and often witnessed, by inoculation for small-pox and cow-pox.
At the time at which Dr Haygarth wrote, very vague and extravagant notions were held on the subject of contagious poisons diffused in the air—of air holding in solution contagious poison, or, as we have determined to call it—Atmospheric Contagion.
It was believed to extend itself to great distances, and there to develope its powers.
His opinions on the subject were, at the time of their publication, quite original; and as they are such as are usually held, to this day, by most intelligent practitioners, the most important will be transcribed here.
In a letter to Dr Percival, on the prevention of infectious diseases, published in 1801, Dr H. says—“I have long thought that there is no subject on which a physician could employ his time and ability more advantageously for the benefit of his fellow-creatures, than in the investigation of febrile contagion, in order to ascertain the laws by which it is communicated, and by what means it may be prevented. It is well known to be the cause of very extensive destruction in the army, the navy, and in large towns.”
“In 1777 I began to ascertain, by clinical observations, (i. e. observations made at the bedside of a patient,) according to what law the small-pox infection, and, in 1780 and 1781, according to what law the febrile infection, is propagated.”—“I found that the pernicious effects of small-pox miasms (that is, airs or vapours) were limited to a very narrow sphere. In the open air, and in moderate cases, I discovered that the infectious distance does not exceed half a yard.”—“Hence it is probable that, even when the distemper is malignant, the infectious influence extends to but a few yards from the poison.”—“I soon also discovered, that the contagion of fevers was confined to a much narrower sphere.”
“You will recollect, my dear friend, that at this time (1781) my attention was much engaged in the investigation of the nature of the small-pox poison. I was struck with the difference of the periods in those two maladies during which the infection remains in a latent state, that is, the interval of time which elapses between the patient’s exposure to the pestilential influence and the commencement of the fever. In the typhus, this period appeared to be much longer than in small-pox.”