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60
untreated, and not isolated until discovered three weeks
after onset of rash, gave rise to only four cases, and these
were all in her own family. No other child in the same
school became infected. Another case (45a), infected while
visiting a case with Smallpox at Enfield during the infectious
period of rash, regularly attended throughout a
period of one month a Mothers' Mission, and infected only
two other cases, both being of her own household. It was
also shown that cases directly infected from case 5 were
themselves more infectious than cases infected from a
different source. It was not discovered where case 5 contracted
her infection.
The mfectivity depends upon the virulence of the infecting
germ, and upon the susceptibility of the person
exposed to infection. There may be a third factor, an
environmental factor which either enhances the infectivity
or lowers the resistance of the exposed person. It may be
in this Bromley epidemic that we were dealing with more
than two sources of infection, each with different powers
of infectivity.