View report page

29
(b) Measles attacks everybody. Some people appear, at any rate during the
greater part of their lives, to be immune from influenza.
(c) Measles protects against a subsequent attack. Many people suffer again
and again from influenza, and the period of protection afforded by an attack may
only last a few months or years.
(d) The infecting power exhibited remains confined within comparatively narrow
limits in measles. It seems to range under ordinary circumstances in London from
a position in which one case on an average infects rather more than one case, to a
position in which one case infects rather less than one case; these variations
are determined mainly by the varying proportions of susceptible children in the
population.
(e) In influenza the pandemic mainly spreads at first among those who mix
largely with their fellows—in workshops, places of business, etc., on the way to and
from work, and in places of assembly. The very young and the aged, who live
sequestered lives, are only attacked at a later stage, and, in them, the trailers find
a special field for their activities. Thus during a pandemic prevalence in London,
the population may be regarded as made up of two sections. In the one—in the
workshops, schools, etc., and in trains, buses, trams and places of public assembly—a
single case of influenza may on the rising wave of prevalence produce from 5 to 10 or
more cases; whereas, in the other section—more or less confined to home life—each
newly affected individual rarely infects more than two or three persons, and may,
indeed, not infect any at all.
(f) There is reason for assuming that the cases occurring early in the influenza
pandemic—i.e., on the rising wave—are apt to run a more rapid, and generally
speaking a more acute, though not necessarily a more fatal, course than those occurring
later. As has been already remarked, the pandemic is both preceded and followed
by epidemics of less rapid development, in which the individual cases run a more
chronic course, and in these trailers the tendency to special involvement either of the
cerebrospinal, gastro.intestinal, or pulmonary system is particularly marked. It is
possible, of course, that when infection results from a very early case the symptoms
show a clinical picture of differing type from that observed when infection results from
a more advanced case.
(g) There are two main phases of influenza—the pandemic phase and the
trailer phase. In the former the adjustment of the relationship between host and
parasite is such as to favour rapid diffusion of the epidemic, rapid implication of one
or another system in each individual affected, and rapid course of the illness ; in
the latter the relationships favour much more gradual diffusion of the disease, local
epidemics tending to affect one or other of the three systems involved in the pandemic,
a more delayed recovery and a marked tendency to the development of relapses.
These considerations may help to render conceivable a closer approximation
in type of the measles and influenza curves, but they by no means completely
explain the varying phases of influenza. In forms of life belonging to the protozoa
and to the lower fungi alternative methods of spread of an organism may be
encountered. In Malaria there are asexual and sexual forms of multiplication.
In Anthrax there is propagation by fission or by spore formation. Again, in the
" rust " of wheat, propagation is effected by uredospores, teleutospores and secidio.
spores ; the first named bringing about the initial rapid spread of the disease early
in the year, the two latter forming " latent " or " carrying on " stages, concerned
in producing extensions of the disease in subsequent years. It may be that in
influenza varying forms of propagation afford explanation of some of the phenomena
presented by diseases of the influenza group. Further, by some such hypothesis
of alternative forms of multiplication, a clue may be furnished which will explain
the way in which encephalitis succeeds influenza, or poliomyelitis replaces cerebrospinal
meningitis; here, in a word, may perchance be sought the reason why the
varying phases of influenza are encountered.
10976 c