This constant destruction and renewal of the body is well known, and it is no doubt this which has given rise to the belief, widely held, that the body renews itself in seven years and that the changes impressed upon it by vaccination endure for this period only.

The truth is that the destruction and renewal of most tissues in the body takes place in a much shorter interval, and, as we shall see, this has nothing to do with the changes concerned in vaccination.

All these activities of the cells vary in different individuals, in different parts and at different ages.
The lesions or injuries of the body which form so prominent a part of disease vary in kind, degree and situation, depending upon the character of the injurious agent, the duration of its action and the character of the tissue affected.

The most obvious injuries are those produced by violence.

By a cut, blood vessels are severed, the relations of tissues disturbed, and at the gaping edges of the wound the tissue usually protected by the skin is exposed to the air, resulting in destruction of the cells contained in a thin layer of the surface.