The aerial convection of small-pox from hospitals / by John C. McVail.
- John McVail
- Date:
- 1894
Licence: Public Domain Mark
Credit: The aerial convection of small-pox from hospitals / by John C. McVail. Source: Wellcome Collection.
Provider: This material has been provided by London School of Hygiene & Tropical Medicine Library & Archives Service. The original may be consulted at London School of Hygiene & Tropical Medicine Library & Archives Service.
6/28 page 4
![As to the ]i()is()n beini^ cai'ried several hundred roods I'roin a liospital, he asked Waterliouse to consider how extreme would be the dilution at this distance, seeinsr that the varioloTis poison is diluted 500 times iiioi-e at the dis- tance of 45 feet than in the patient's chamber. Then Ha}-- garth goes on to argue that negative evidence is incom- parably stronger than affirmative evidence. He calculates that, in any place visited by small-pox, if one has escaped the disteujper, it is 19 to 1 he has not been exposed to the infection; and that if three in a family have e.scaped, the probability that they were not all three exposed is 8,000 to 1.* Therefore, he holds that the escape of persons within a specified distance from a source of infection is almost infinitely sti'ong evidence that the infection could not liave been carried so far from its soui'ce. And now, in the history of this controversy, we come to the remarkable fact that Waterhouse in reply expresses agreement with this doctrine of Haygarth's as to the value of negative evi- dence.Though he continued to defend the various ex- amples he had given, yet in yielding this point he j'ielded his Avhole case, and it is not surprising that Haygai'th, just at the end of his book, is able to relate that Waterhouse, having .subsequently had experience of a small-pox epi- demic in Boston, had arrived by negative evidence at the conclusion that the virus was not atmospherically convey- able, as he had originally believed. Unfortunately, this letter of Waterliouse's arrived too late for publication by Haj-garth, so that we have no details as to the negative evidence in question. It is, howevei, an interesting indica- tion of the nature of the proof that Haygarth relates (from Waterliouse's letter) how, in this particular epidemic, a great number of failures occurred in conveying the disease hy inoculation, even after four or five successive attempts, and how verj^ few were attacked in the ordinary waj^ although many Avere in the same room with tho.se * This calculation starts with the opinion (ba.sed on facts adduced by various wiicers) that about 5 per cent, of all persons born are. insusceptible to sniall-pox. Jf 19 out of everj' 20 persons are su.sceptiblo to suiall-pox by aerial infection, the chances are 19 to 1 that an expensed person will contract it; and if any person has not taken .sniall-pos, the chances are 19 to 1 that he has not been exposed to it. If two persons in a family do not contract the disease/the chances are 19 x 19, or say 20 x 20 = 400 to 1 that they have not been exposed ; and if three persons in a family do not contract it, the chances are 20 x 20 x 20 = 8,000 to 1 that they have not been exposed to it. t Negative evidence is, of course, valualile when comjtared with affirmative evidence, as indicating the infreiiuency or frerpiency of aerial convection, and it i.4 to be hoped that as facts accmnulate in regard to tin- wliole t|uestion, full use will be made of the comjiarison ; but Haygarth's doctrine was iti efl'ect that negative eviilence is destructive of allirmative evidence.](https://iiif.wellcomecollection.org/image/b21361861_0008.jp2/full/800%2C/0/default.jpg)
