All authorities agree that asthma is much more frequent among males than females. Of Théry's cases, 60 were females and 80 males. The more recent statistics of Salter show that the males exceed the females in the proportion of two to one. This undue frequency of a purely nervous disease among males appears at first to be at variance with the generally-received opinion that such affections pertain rather to the female sex; but on investigating the ages at which the attacks first come on it will be found that between the fifteenth and thirtieth years—that is, during the period when sexual function is most active—the proportion is reversed, females being attacked much oftener than males.
Asthma occurs more frequently in childhood than at any subsequent period—a fact which may be explained by the great susceptibility of young children to catarrhal affections of the air-passages and to the frequent occurrence at that age of measles and whooping cough (Salter). Of 225 cases collected by Salter, 71 occurred before the tenth year, and of these, 10 began during the first year, the youngest of them being only fourteen days old at the time of the attack. From ten to twenty it occurs less frequently than at any other period of life, but from that age to the fortieth year there is a steady increase in the number of cases. During the next decade, from forty to fifty, the disease diminishes in frequency, and from that period on the number of cases continues to grow smaller and smaller with advancing years, comparatively few commencing after the fiftieth year.
The following tabular statement, compiled by Salter, shows the comparative frequency of asthma during the various periods of life:
| From | 1 to 10 | years, | 71 | cases. |
| " | 10 to 20 | " | 30 | " |
| " | 20 to 30 | " | 39 | " |
| " | 30 to 40 | " | 44 | " |
| " | 40 to 50 | " | 24 | " |
| " | 50 to 60 | " | 12 | " |
| " | 60 to 70 | " | 4 | " |
| " | 70 to 80 | " | 1 | " |
These figures demonstrate the fallacy of the popular idea that old people are especially liable to asthma. Its prevalence during the later periods of life is due to the fact that while, on the one hand, the affection rarely causes death, on the other it is scarcely ever curable except during childhood, and thus the cases contracted at different ages accumulate and form a large aggregate as life advances.
Those cases occurring in childhood and late in life are likely to be associated with more or less bronchial catarrh, while those which come on when the body has attained its fullest development are almost invariably purely nervous in character.
The period of life at which asthma commences is an important element in the prognosis of the disease, the cases occurring in early childhood being likely to end in recovery, while those coming on later in life are exceedingly protracted in their course and liable to lead to organic diseases of the heart or lungs.
Asthma does not appear to be influenced by the seasons, some authors claiming that it is most frequent in summer, while others maintain that the greatest number of cases occur in winter.
Exciting Causes.—Bronchial asthma being a neurosis of the pneumogastric nerve, its exciting causes may be divided into those which act upon the nerve directly, and those which are reflected from more remote parts or organs.
In the first class the irritant may act upon the nerve at its origin in the medulla oblongata or upon some part of its continuity. Various poisons, organic or inorganic, when introduced into the system may so change the character and composition of the blood as to interfere with the nutrition of the respiratory centre, and thus cause more or less embarrassment of respiration; but the attacks of dyspnoea due to these causes are more continuous than those of ordinary asthma, and are wanting in many of the symptoms which we have described as characteristic of that disease. These forms of dyspnoea are usually the result either of some constitutional disease or of some poison introduced into the system, both of which act by diminishing the proportion of red corpuscles in the blood. Of this we have examples in the dyspnoea sometimes observed in syphilis and malarial fever and in lead and mercurial poisoning—the so-called a. saturninum and a. mercuriale. It is true that there have been instances in which the paroxysms of asthma have come on at regular intervals and have yielded to quinine, but it is not regarded as proved that such cases were due to malarial poisoning (Sée).