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If Diphtheria may be defined as any pathological condition,
local or general, due to infection by specific Diphtheria organisms,
then Diphtheria is "latent" when such pathological conditions
are unaccompanied by obvious illness or by symptoms sufficiently
characteristic to be recognisable as Diphtheria. We come in
contact with three kinds of cases that may be classed in this
group:—
1. Persons who are in poor general health associated with
some abnormal condition of the throat, nose or ear.
2. Cases of local Diphtheria lesions, but presenting no
general symptoms of ill-health.
3. Cases with no local lesions and no departure from
normal health.
..
Bacteriologically, Diphtheria is latent when the specific germs
are present unaccompanied by any obvious pathological change.
The infective agents are held in check by what we call vital
resistance on the part of the tissues, or diminished virulence on
the part of the specific microbe.
The nature of the immunity enjoyed by the carriers of
virulent bacilli is obscure, and the only method as yet available
to meet the requirements cf prompt administrative action is not
able to differentiate between the comparative virulence of the
bacilli.
The power of the unrecognised case and the positive contact
to infect has been amply demonstrated, though the accident is
much more rare than might be anticipated from the large
number of mild cases and infected contacts in the community.
The number of possible infectors is also reduced when we
consider that it is only likely to be persons harbouring virulent
bacilli that are capable of infecting others with disease-producing
bacilli.
When the bacillus gains a lodgement in the throat, it
appears to remain for a varying length of time, whether it