There is, however, in living things no definite type for the normal.

An ideal normal type may be constructed by taking the average of a large number of individuals; but any single individual of the group will, to a greater or less extent, depart from it.

No two individuals have been found in whom all the Bertillon measurements agree.

Disease has reference to the individual; conditions which in one individual would be regarded as disease need not be so regarded in another.

Comparisons between health and disease, the normal and the abnormal, must be made not between the ideal normal and abnormal, but between what constitutes the normal or usual and the abnormal in a particular individual.
The conception of disease is so inseparably associated with that of life that a brief review of the structure and properties of living things is necessary for the comprehension of the definition which has been given.