In the study of the malignant cancerous thickening of the tumor-like nature in a cat’s mammary gland with the subsequent staining of the pathological material by the improved method according to H. C. Gram, a category of rod-shaped bacteria of the cord form was found. It was noted that these microscopic (visible in an optical microscope) bacteria are able to cluster into dense formations and, with an increase in the temperature of the external environment, form dense living “ropes” and “bundles” from tens of thousands microbial individuals. This gives them the opportunity, getting and reproducing in a favorable environment, thereby increasing their population in quantity, inside the living body of the diseased animal organism, pulling the affected parts of the body and its internal organs and tissues with cord-like overlays, leading to their microbial suffocation. The flow of nutrient and protective elements with the blood stops and the transmission of the nerve impulse is disrupted, which immediately leads the affected parts of the body to tumor-like growths with their subsequent necrosis (necrosis). Dead tissue, and in our case, the mammary gland of a cat, is an excellent breeding ground for the development and reproduction of colonies of ciliated microbes in it, capable of forming spores in case of unfavorable factors for their vital activity. Reproducing and becoming sexually mature individuals, within 20 hours (approximately, in time, less than one day), cord-like microbes begin to immediately form living microbial bundles of various thicknesses and sizes. Covering a certain separate area of living tissue with a microbial “loop”, squeezing the affected parts of a living body – blood vessels and nerves, thereby slow down, and in some cases completely stop blood flow and nerve impulse. All these negative factors gradually lead to the extinction of the vital tone of muscle and nervous tissues, which inevitably leads to cell death and the growth of a dense tumor-like neoplasm. Malignant cancerous tumor of domestic animals does not arise from the previously voiced chemical theory of Justus von Liebig, but because of the presence of a living biological pathogen – ciliate rod-shaped formed bacteria – inside the living parts of the body. Staining according to the improved method of H. C. Gram with blue dye (methylene blue) instead of a violet dye gentian violet, and a red dye (safranin) instead of Pfeiffer's carbolic fuchsin, allowed under the optical immersion system of a light binocular microscope with a magnification of 1000 times, to see special cord-like, wriggling loop-shaped forms, consisting of a large number of rod-shaped microbes. They are able to create inseparable twisted chains and “loops” from their tiny microbial bodies, which gives grounds to assert their significant role in the occurrence of malignant cancerous neoplasms.