Lots of things cause the symptom or disorderof cancer, from radiation, to overexposure to certain chemicals, to certain viri (e.g., the HPV virus).
Cancer begins in your cells, which are the building blocks of your body. Normally, your body forms new cells as you need them, replacing old cells that die. Sometimes this process goes wrong. New cells grow even when you don't need them, and old cells don't die when they should. These extra cells can form a mass called a tumor. Tumors can be benign or malignant. Benign tumors aren't cancer while malignant ones are. Cells from malignant tumors can invade nearby tissues. They can also break away and spread to other parts of the body.
Cancer is not just one disease but many diseases. There are more than 100 different types of cancer. Most cancers are named for where they start. For example, lung cancer starts in the lung, and breast cancer starts in the breast. The spread of cancer from one part of the body to another is called metastasis. Symptoms and treatment depend on the cancer type and how advanced it is. Most treatment plans may include surgery, radiation and/or chemotherapy. Some may involve hormone therapy, biologic therapy, or stem cell transplantation.
Even that is an oversimplification. There are over 200 different types of cells in humans. There are four main types of lung cancer, IIRC. At least one type of bacteria, Helicobacter Pylori, the bug responsible for about ninety percent of gastric and duodenal ulcers, is also thought to be the cause of MALT lymphomas, etc.
Saw the replies to my comment just now and thank you neverdem for the clarification.
The fact that cancer cells have a mind of their own is a clear sign that they had been hijacked and their genetic material completely transformed. And only viruses are known to do that.
Things like radiation, chemical carcinogens, or even stress probably damage or weaken the cells (and the immune system) and make them susceptible to all kinds of attacks, both bacterial and viral. What we believe to be a disorder or systemic may actually be caused by some distinct pathogens, such as the discovery of H. pylori in ulcers you mentioned.