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In studying scarlatina it is evident that little account need be taken of school children's ages
with regard to the numbers protected by previous attack—a striking contrast to ; measles where
this factor is so very important.
An enquiry into the method of propagation was pursued by Dr. Stevenson, 132 cases altogether,
chiefly but not exclusively the school children of five schools being investigated both at school
and at the child's house. In addition to these a considerable number of outbreaks were inquired
into at the school only.
Incubation period.—Amongst the 132 cases thirty occurred of secondary cases in
households, under circumstances pointing with some probability to infection from the primary
1905 6
MEASLES. 1906-7
Diagram XIV.
case. Certainty in any individual caae is unattainable, but the general teaching of a considerable
number, if it points strongly to any particular period, cannot well mislead. If all the cases were
coincidences, there would be as many secondary cases in the third week after the primary case as in the
first. Of the thirty cases, however, 21 pointed to a period of five days or less and only two to a
minimum of over a week. The conclusion seems inevitable that, in most of the 30 instances, the
secondary was certainly infected by the primary case, with an incubation period commonly of five
days or less.
Infectivity and Susceptibility.—In nearly all of these instances the primary must
have infected the secondary case, within the first day or two of illness. It was the removal to
hospital of the primary case, and disinfection of the premises, which enabled a minimum limit to be