Recently, some readers have written to us asking whether the appearance of cancer cells in the body is equal to having cancer. To answer this question, we should first understand what cancer is. Cancer, also known as malignant tumor, is a type of disease caused by the malfunction of the mechanism regulating the growth and proliferation of human cells. Its main characteristics are unlimited growth of uncontrolled cells, invasion of surrounding normal tissues, and distant metastasis. These characteristics determine that cancer is a cellular disease, that is to say, in a broad understanding, cancer cells found in human body are equivalent to having cancer. However, this understanding is not exactly the same as the cancer found in our clinical treatment activities in the usual sense, because cancer is still a very time-series disease, which has a complete process of occurrence and development, and only the latter quarter of the whole process can be perceived by us. Generally speaking, there are about 109 cancer cells in 1 gram size of cancer tissue, and it takes 30 times of multiplication from 1 cancer cell to reach this number. The growth rate of different cancers varies greatly, with the fastest-growing cancers multiplying in less than 3 days and the slow-growing ones up to 600 days or more. Cancer cells proliferate to about 1 gram in size before they can be detected by existing imaging methods, so the clinical definition of cancer often refers specifically to the quarter stage thereafter.