View report page

29
Further, is the question as to whether the altered proportion
of susceptible to immune in the population does not influence
the incidence. If each case is considered as a focus of infection
in a larger susceptible community, the additional cases
might be expected to mount up not by arithmetical, but by
geometrical progression. While this might be expected however,
it is difficult to see where the factor comes into operation.
It is equivalent to saying that a child population of 18,750 is
having more cases because it is distributed amongst a general
population of 50,000 instead of a population of 75,000. If,
therefore, this effect is apparent the cause must be looked for
not in the fact that there is this aggregation of susceptible
population, but rather to the fact that there is a diminution in
proportion of the insusceptible population.
The spread of Infectious Disease is dependent upon a number
of factors which include the host (his susceptibility or resistance)
the infecting virus (its virulence and pathogenicity) and
the rate of infection. If the interaction between virus and
susceptible host results in the suppression of the virus, no
disease appears in the host, and further than that, the degree
of immunity established is such that after successfully overcoming
an attack, the host is left in a position that he is less
susceptible to infection than he was before the attack. It is due
to his successfully resisting repeated small sublethal doses of
infection that man develops his acquired immunity to disease.
Should, however, the velocity of infection of the early invasive
dose be greater than can be dealt with by the defensive
mechanism, the host develops an attack of that particular
infection. It can be seen therefore that while a case of infectious
disease is a possible source of other cases, it is also
acting as an immunising agent to other susceplibles and so
producing an insusceptible population.
In addition to the spread of infection by actual cases of
disease, to some extent they are spread by carriers, who may be
convalescent or contact carriers, and are persons who are immune
to the disease. These carriers in the same way, besides being
potential sources of infection are also, through their disseminating
small doses of infecting agent, actings as immunising
agents through the community. In an ordinary population it
is unknown how far this immunising effect on the population
is produced by actual cases of disease and how much by immune
carriers. If the latter, then where these immune are poorly
represented one would expect that those susceptible would
have fewer opportunities of being immunised and therefore
more likelihood of being attacked by the massive dose by close
contact with a case before immunity is established.
A further effect of the relative absence of immunes would be
on the organism. Bacteria passing from one to another