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101
(a.) Simultaneous infection of the secondary and
primary patients, the later development of the
secondary case being due to a longer incubation period
by reason of some personal idiosyncrasy of the patient;
(6.) Infection derived from the primary case at
some time between the onset of the illness and the
inception of isolation, or removal to hospital, or, when
the primary patient is kept at home, to imperfect
isolation;
(c.) The release of the primary patient from
isolation, or his return from hospital, before all
infection has left him;
(d.) Failure of the method of disinfection, either
through inefficiency of the process or material used,
or the omission to disinfect some particular article;
and
(e.) Independent sources of infection.
In an inquiry such as this, the three first and
last possibilities ought to be excluded before any
particular case of spread of infection is assigned
to the operation of the fourth. A standard period of
time for the incubation of each disease must, therefore,
be determined upon, and such standard should
include the longest periods recorded with some
frequency, but not what may be called exceptional
cases of prolonged incubation.
Without discussing the varying lengths of incubation
periods, it will suffice to say that the
incubation period of scarlet fever is frequently as