The old idea was that tuberculosis was inherited. People got this idea because they saw the disease occur so frequently in families, and saw it run through two or three generations. We now know why this happens. It is not because tuberculosis is inherited but because it is communicable in a peculiar way. For communication of the disease a long intimate association is necessary, such as most frequently exists in the family, and therefore the disease is conveyed oftenest along the family tree. The intimacy which is necessary for the spread of tuberculosis may also exist outside of the family in places of employment, and the disease is sometimes conveyed in this way. Tuberculosis is never transmitted from the parent to the offspring in the true sense of heredity.

Sometimes a child is born with the disease, but this is because the mother has the disease so far advanced and so widely distributed in her body that the child gets it by direct contact. The child is then born with the disease and usually dies shortly after birth. Even this occurs very rarely, and so free is offspring of tuberculous parents from the disease that in some parts of the world tuberculous cattle are used for breeding purposes, the young being separated from the parents immediately after birth and thus brought up in perfect health.