warring forces of evolution and retrogression, and the species in respect to inborn immunity will remain stationary. For which reason it is that complete inborn immunity is not attained against such very common diseases as measles and chicken-pox, which almost every individual in Europe suffers from, but which are only fatal to a very few—to those, speaking generally, who have reverted, by the lapsing of inborn variations, to the ancestral condition of non-immunity.
As regards acquired immunity: it is a well-known fact, that duration of the attack in many zymotic diseases is limited in the individual; that is, if the individual attacked does not die, he recovers within a certain pretty definite period of time, which is short in some diseases,—e.g. measles,—longer in others,—e.g. typhoid,—and longer still in a third class—e.g. syphilis; but certain other diseases—e.g. tuberculosis and leprosy—may persist in the individual for an indefinite time. It is also well known that in all diseases which have a limited duration in the individual, one attack confers for a shorter or longer time complete or partial immunity against subsequent attacks. This is the second kind of immunity, the kind which is not transmissible, but is acquired anew by each individual; for example, one attack of small-pox is usually followed by complete or partial immunity from subsequent attacks of that disease, the individual attacked, in the vast majority of cases, not taking the disease again, or taking it only in a mild form a considerable time after the first attack. Never, or so very rarely that it is regarded as a medical curiosity, is one attack closely followed by another.
Now it is plain, if such a disease as small-pox or measles persisted for an indefinite time in an individual, or if an individual were subjected to repeated attacks of the disease, that either condition would be equivalent
