Ancient Chinese observed the nature deeply and from these things they formulated some universal principles. Basics of their theory medicine are a part of their whole philosophy.
The first rule they formulated is that everything is a different manifestation of the same energy; called
KI, (Chi in Chinese). Characteristics of Ki are the movement and the transformation, things are always changing, and only the change never changes.
From these observations they also created the two majors rules used in medicine, The
Yin and Yang theory and the Five elements theory.
They both speak about the different qualities of the Ki.
As human is a part of the whole, Ki is also flowing in his body (it's flowing in some channels in a more concentrated manner, that's what we call meridians ).
In this point of view all sickness comes from energy stagnation (physical or mental). Generally speaking, something is wrong when communication is dead, that is not connected to the others. It's true for human being as well as for our body.
If the deep roots are in Ancient China, the modern form of Shiatsu has been strongly influenced by the two majors figures of the twentieth century. The first one was Tokujiro Namikoshi who fused techniques from traditional Japanese bodywork systems, with a Western understanding of human anatomy. The second one was Shizuto Masunaga, who interpreted the traditional Chinese medicine in terms of Western physiology and psychology.
Masunaga Sensei created the Iokai Shiatsu and his style is referred to as Zen Shiatsu. Nowadays, there are a lot of different shiatsu schools and each of them has their own style but more or less, they all refer to these two major masters.