Carcinogenesis, i.e. the formation of tumours in a hitherto healthy organism, is a complex process, usually taking place gradually and taking years. External factors play an extremely important role here: chemical (e.g. formalin, food preservatives) or physical (e.g. radiation), infections with oncogenic viruses (e.g. Human Papilloma Virus H. pyloriV 6, 11) and others, which are already well established.