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pools or of sewers; if typhus fever breaks out, there is ground
to suspect that the population among whom it appears are
overcrowded, their houses filthy and insufficiently ventilated.
But it does not necessarily follow that the presence or the
spread of such diseases proves local insalubrity: many of them
are infectious, and if introduced into a crowded neighbourhood
will spread in it however otherwise healthy that neighbourhood
may be. The prevalence of any of them, however, in a
markedly aggravated form, or for an unusually long period,
renders the presence of local causes of insalubrity exceedingly
probable."