During recent years it has been discovered that the symptoms of phthisis and scrofula are due to a microbe, the tubercle bacillus. It appears, however, that this bacillus cannot gain access to, or multiply in, the tissues of a healthy and vigorous man or woman; most of us probably have often carried this micro-organism within the mouth or stomach, and though our gastric juice has not been able to destroy it, as is the case with so many of our invisible foes, it has been unable to pass into our blood or lacteals. Dr. Woodhead puts this fact strongly.[14] He says: “A perfectly healthy individual, placed under favourable conditions as regards food, fresh air, and exercise, is never attacked successfully by tubercle bacilli, the active, vigorous tissue cells being perfectly able to destroy any bacilli that make their way into the lungs, the pharynx, or the intestine.” It appears, too, that a certain type of individual is readily attacked by this microbe, while the normal individual, debilitated though he may be by unfavourable external conditions, falls far less readily a victim. As Prof. Sir Lauder Brunton[15] remarks: “We are constantly meeting with persons belonging to very consumptive families who escape the disease by living under conditions where the bacillus tuberculosus is likely to be absent. On the other hand, persons such as nurses are in all probability frequently inhaling the microbe, and yet are not attacked by the disease. In the first case immunity is probably due to the absence of the seed, notwithstanding the favourable condition of the soil; in the second it is due to the barrenness of the soil, notwithstanding the presence of the seed.” Inasmuch as phthisis is markedly hereditary, we may look upon the _type_, not the disease, as being transmitted. A phthisical type of person is one who comes of a family liable to fall a prey to this microbe, and he is recognisable by many distinctive characteristics of hair and complexion, and by qualities of temperament, feature, and figure. Sufferers from phthisis are prone to other diseases, such as pulmonary and bronchial attacks, so that over and above the vulnerability to this one form of microbe they are to be looked upon as unsuited not only for the battle of life, but especially for parentage and for the multiplication of the conditions from which they themselves suffer.