The efficient control of the spread of yellow fever is a matter of such vast practical importance, both from the hygienic and commercial point of view—not only for the countries where this disease prevails as an epidemic, but also for those in which, after importation, it may assume epidemic proportions—that it has seemed appropriate to bring together in this paper a summary of the work thus far accomplished by the United States Army Commission 1 on the Island of Cuba, during the years 1900 and 1901, in order that English and Colonial readers who have not, perhaps, had access to the original contributions published in different American journals, may be able to form an intelligent opinion concerning the permanent value of this work. It will also afford opportunity for recording the more recent confirmatory observations made by others concerning the mode of transmission of yellow fever discovered by the Commission, and for calling attention to the results already obtained by the U.S. Army Medical Department in the suppression of this disease, especially in the city of Havana, through the enforcement of sanitary measures based on these later researches.

The numerous additions to the group of Streptothrices which have been recently made, and the close resemblance which the majority of these newly described forms bear to one another, render it by no means an easy task to assure oneself that a freshly isolated member of this group is one that has not previously been met with and described. A curious fact also, which soon strikes anyone who investigates the literature relating to this group of micro-organisms, is that in very few of these has the particular form described by one author been met with by another. While great interest to the bacteriologist naturally attaches to any new species, whatever its source, the group of those which are pathogenic in their action on man, on animals, or on both, are, ipso facto, of more general importance. To this last category belongs the new member of the group which we have isolated from a fatal case of lung disease and pericarditis in man.

The above experiments show that neither the mineral acids nor permanganate of potash are reliable disinfectants of mud floors against the plague bacillus, even when used in solutions which are many times as strong as are required to destroy the plague bacillus in test-tubes in 15 minutes. This is doubtless owing to the former being neutralized by the alkalies contained in the earth and in the cowdung used for leeping, and to the latter being precipitated and rendered inert by combining with organic matter, which is plentifully present in the leeped mud floors of rooms inhabited by the poorer classes in the large towns and villages of India, which are so often affected by plague.

Of the disinfectants which are sufficiently cheap to be employed on a large scale, there remain phenol and perchloride of mercury in acid solution, the former of which has given satisfactory results on mud floors in a strength of 1 in 50, while the latter is effective in a strength of one part of the mercury salt and two parts of strong hydrochloric acid in 500 of water. If we compare these strengths with those respectively required to destroy plague bacili in 15 minutes in test-tubes, we find that 1 in 750 of phenol and 1 in 20,000 of acid perchloride of mercury solution are effective under the latter circumstances. Thus it appears that in order to be sure of killing micro-organisms of as little resisting power as the plague bacillus on the surface of mud floors, 15 times as strong as solution of phenol and 40 times as strong a one of acid perchloride of mercury as are effective against the plague bacillus in test-tubes in 15 minutes must be used. Thus in proportion to its action on the plague bacillus in test-tubes, phenol is nearly three times as effective on a mud floor as is acid perchloride of mercury. This fact clearly indicates that a portion of the mercury salt is rendered inert by combining with certain constituents of the mud floor, probably albuminous substances, and this may explain the at first sight somewhat surprising failure of a 1 in 1,000 mercurial solution efficiently to disinfect the mud floor. However, when the strength of this solution was doubled, the acid being also doubled, satisfactory results were obtained with the mud floor tested, and the micro-organisms recovered after its use proved to be much highly resistant than the plague bacillus, while they were fewer than even after the use of 1 in 50 phenol. Nevertheless, the neutralizing action of mud floors on the mercury solution will doubtless vary considerably in different places, and it is easy to conceive that in some instances this variable and unrecognisable factor may be considerably greater than in the case of the particular floor used in these experiments, and that even a 1 in 500 solution might be ineffectual under such circumstances. On the other hand, phenol appeares to be free from this objection; and as it gave as good results in a strength of 1 in 50 as the acid perchloride did in one of 1 in 500, it must be considered the safer disinfectant in this strength (1 in 50) in the case of mud floors. In the case of impermeable floors either a 1 in 100 solution of phenol or a 1 in 1,000 one of acid perchloride of mercury is efficient, the latter being somewhat the more powerful of the two, which advantage is probably more than counterbalanced in practice by the disadvantage of using two different disinfectants, if the conclusion that phenol is the better in the case of mud floors is accepted. both 1 in 50 phenol and 1 in 500 acid perchloride of mercury cost about one penny a gallon, the former being very slightly the cheaper of the two in such price-lists as I have been able to consult.

The variable effects of mud floors in partially neutralizing the disinfectant power of perchloride of mercury solutions may possible account for the different estimations of the value of disinfection with this agent in different places and provinces, and also for the failure of the very extensive operations of the year 1900 to prevent the recurrence of plague in the same houses and bustees during the recrudescence of the disease in Calcutta early in 1901. It must also be remembered that the above-noted strengths of disinfectants only act on the superficial layer of floors, etc., and have no influence on the presence of rats, by which, it is now pretty generally admitted, plague is in some indirect way spread—an important factor which must not be lost sight of in estimating the probable advantage of measures of disinfection. Still it is doubtless a good thing to destroy any plague bacilli lurking in a room which has been inhabited by a plague patient, and the practical outcome of the present enquiry is that the perchloride of mercury in hydrochloric acid must be used in at least double the strength that was provisionally recommended by the Plague Commission, and which has up to now, I believe, been very generally relied on in Calcutta. It is still better to substitute a 1 in 50 solution of phenol in every case when mud or earth floors or courtyards form any part of the area to be dealt with, as will be the case in the great majority of instances in which disinfection for plague is required.

The present absence of any Statutory regulations as regards the discharge of sewage and other pollutions into tidal waters has allowed many estuaries in this country to be converted into receptacles for the filth of the sea-port and manufacturing towns situated near the mouth of the larger rivers. Only by an order of the Local Government Board, made after local enquiry and on sanitary grounds, can an estuary be declared a stream and thus brought under the various Rivers Pollutions Acts.

The outbreak about to be described raises issues of practical importance which appear to make it worthy of record, in spite of the fact that owing to fear of injury to a particular dairyman it was impracticable to complete the evidence by making inquires at every house supplied with milk by him.

Finally I wish to draw particular attention to the following facts:—

1. The striking diminution in the case-mortality at the Mile End Hospital which followed the systematic use of antitoxin.

2. The subsidence of the epidemic during the season when diphtheria is commonly most prevalent.

3. The opening of the schools in September without any increase in the prevalence of the disease.

4. The persistence of diphtheria bacilli for long periods in certain of the convalescents and contacts.

5. The success of preventive measures based upon the belief that diphtheria is spread mainly by personal contact, and through the inter-mediation of healthy persons and other who by the means usually employed are not recognised to be suffering from the disease.

In this outbreak of diphtheria certain individuals suffered from diphtheria, and others from sore throat probably diphtheritic. These individuals obtained their milk supply from two cows. Members of one household who did not drink milk, or who used it only after sterilisation, escaped infection. The cows yielding the milk were found to be suffering from an eruptive disease of the udder, and both from the lesions and from the milk cultures of virulent diphtheria bacilli were isolated. The pathological condition in the cows preceded, by a short interval, the onset of the disease in the patients. Having regard to Power's epidemiological investigations, and to Klein's experimental work, this observation was of the greatest interest and naturally suggested the hypothesis that the lesions were due, primarily, to a specific diphtheritic infection of the cow. Further investigations weakened this view, for it must be noted:—

(1) That in a cow intentionally submitted to infection from the diseased cows and in which there occurred the eruptive condition of the udder, neither in the vesicular nor in the ulcerative stage of the disease could diphtheria bacilli be demonstrated.

(2) That in calves infected with the eruptive disease no diphtheria bacilli could be demonstrated.

(3) That in a calf, in spite of the fact that it had received 10, 000 units of diphtheria antitoxin, the vesicular eruption was experimentally produced. The last is probably the strongest point in support of the dual nature of the condition in the cows.

It is conceivable that pathological lesions in the cow such as those described, if infected with the diphtheria bacillus, might form a suitable nidus for its growth and permit of the infection of large quantities of milk over a considerable period. Though we have as yet no evidence on the subject it is possible that a profound change in the virulence of the diphtheria bacillus for the human subject might be effected by such passage through the cow1.

The disgusting habit the milkers in this part of England have, of spitting on their hands before milking, would easily account for the infection of the lesions, even in the absence of obvious diphtheria in the cowman; knowing, as we now do, that apparently healthy individuals are not uncommonly the hosts of the diphtheria bacillus.

The experiments made with the view of ascertaining whether the eruptive condition was genuine cowpox, are opposed to that view; for two calves, and one cow successfully infected with the eruptive condition and subsequently vaccinated with vaccine lymph, developed a typical vaccinia.

The somewhat limited time and material available have prevented us from investigating the matter so fully as we should have wished, or on a scale proportionate to its interest and practical importance. We hope at some future time to go further into the matter.

Our best thanks are due to Mr Sidney Villar, F.R.C.V.S., for bringing the outbreak to our notice, and for his help in the veterinary side of the question; and to Dr Carson Smyth for his kindness in allowing us facilities for taking cultures from the throats of his patients.

The following notes are to serve as a postscript to the paper which appeared under the above title in the preceding number of the Journal of Hygiene (pages 1—42).

The primary object of these papers is to attempt to give a brief review of the present state of opinion regarding the many questions involved in the subject of immunity,—a review intended chiefly for those whose work is concerned with other fields of hygienic research. The carrying out of this aim has necessitated the re-statement of many facts familiar to bacteriologists, in order that the continuity of the discussion might be maintained. Wherever it has been considered advisable the details of the data which have formed the basis of theory have been given, but as a rule the leading results have been alone dealt with, partly because details might obscure the general principles it was intended to emphasise, partly because considerations of space have made an exhaustive treatment of the subject impossible.