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much nearer to our doors than was formerly the case. It is a novelty in sanitary history for a patient, whose attack of smallpox showed itself on his arrival in London, to have received his infection in Manchuria. Yet that has happened. Smallpox has many more resources to draw upon than formerly; and when the disease breaks out among us improved internal locomotion offers greater opportunities for its spread.

A review of the past prevalence of smallpox in this country shows that it has come in cycles. It was very prevalent in 1871, in 1877, in 1881, in 1884, in 1893, and in 1902. Nine years of comparative quiescence elapsed between the maxima of 1884 and of 1893, and nine years between the maxima of 1893 and of 1902. A further nine years on brings us to 1911, which has happily passed without an epidemic. It is now eleven years since the last maximum and, in the autumn of 1913, we are still free from smallpox. How long this freedom will continue cannot be said. It is impossible to prophesy. Only two things are certain. Susceptible material is increasing, and infection is continuing to come in. Though the infection of case after case be extinguished, sooner

or later a spark will come in contact with a collection of highly combustible material; in that event the blaze will be sudden, and it will be fierce.

Whatever the future has in store, our part is plain, namely, to be forearmed. It is to assist in that object that these pages have been written.

CHAPTER II

SOME POINTS IN THE NATURAL HISTORY OF

SMALLPOX

In making our plans against smallpox, we shall do well to begin by dwelling on one or two points in its natural history which bear especially on its prevention. First as to the incubation period. This is singularly constant. Twelve days is the rule. Variations occur; but in the majority of cases signs of illness show themselves with remarkable conformity to the expected time; smallpox has a more regular incubation period than any other infectious disease. It is convenient to keep a type of the chronology of the disease in mind. for reference.

For example:

Sunday, January 1st.

Friday, January 13th.

Exposure to infection.
Onset of fever, headache,

other pains, malaise,
vomiting.

Sunday, January 15th. First appearance of papular rash.

Tuesday, January 17th. Rash begins to be vesicular. Thursday, January 19th. Rash begins to be pustular. Monday, January 23rd. Scabbing stage may begin. Monday, February 6th. Scabbing stage may end.

These dates are of special importance in administration, and the earlier ones form a sequence which may be counted upon with some confidence. If a susceptible person be thoroughly exposed to infection on a Sunday, for instance, the following Friday week will not pass without signs of the beginning of his attack.

Next, as to the infective capacity of smallpox. We all know how highly infectious it may be. But the striking power of different cases and of different groups of cases varies in a remarkable manner. At a time when London was completely free from smallpox-it was in the year 1897 or 1898-I recall, for instance, a case of a woman

having an attack of smallpox which later became almost confluent; she was stated to have sat for a long time in an out-patient department of a hospital awaiting examination. The rash was then well out, and the patient thoroughly infectious; there must have been many people in contact with her before her illness was recognised. As a consequence, a small outbreak might have been apprehended, and it would seem that in any case secondary cases were inevitable; but no secondary cases occurred, or at any rate came to knowledge, except that of the patient's husband.

On the other hand, there are recorded cases whose progress, from shop to shop or from street to street, has been marked and traced by a perfect trail of secondary cases to which they have given rise. This high degree of infectivity is apt to proceed from severe or hæmorrhagic cases. Similar instances of this kind of contrast will come to the minds of those who have had experience of smallpox.

The same kind of phenomenon seems to hold true of smallpox in the bulk. At the beginning of an outbreak, cases in general are apt to be more severe and to have a higher average of infectivity than those which occur during the decline of

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