How does cancer happen? As discussed in the article CANCER, normal cells grow and divide in a controlled way to produce more cells for keeping the body healthy. And, these cells get old or damaged, they die and are replaced with new cells. However, when mutation happens, old cells do not die and new cells continue to form, and finally forming a mass of tissue, i.e. a tumor. Tumors can be benign or malignant. Cells in benign tumors do not spread to other parts of the body. While, cells in malignant tumors can invade nearby tissues and spread to other parts of the body. The spread of cancer from one part of the body to another is called metastasis.
What are the common causes for cancer? In that article, we have also discussed the causes and risk factors for the cancer development. Exposure to certain chemicals such as asbestos, acrylamide, alcohol, tobacco else increases the odds of development cancer significantly. Genetics and body conditions also affect the odds of cancer development. Definitely, obesity is one of the causes for cancer development, or at least obesity facilitates cancer development by weakening our body defense. There are also specific causes (or risk factors) for specific cancers. For example, a harmful mutation in BRCA1 or BRCA2 is the cause of breast cancer. _____________________________________________________________________________________ A Special Case In addition to the common cancer causes that we know, there are some other causes that we may not be aware of. This section is to discuss how a daughter accused cancer from her mom:
“Since 1866 there have been some 17 documented cases (including the present study) of a baby developing the same cancer as its mother, suggesting that the mother’s cancer cells had metastasized to the developing foetus.” – Nature.com, October 13, 2009. 
British researchers have found that a 28-year-old mother, who died of leukaemia, passed on her malignant cells to her daughter in the womb. 
Samples of the baby's blood and those of the mother examined by Mel Greaves, of the Institute of Cancer Research (ICR) in Surrey using "genetic fingerprinting" showed that they both contained exactly the same rogue cancer cells. Individual cancers develop genetic mutations randomly and no two cancers are the same, unless they have the same source. And in this case, the mother and daughter had the same mutations, thus, the cancer must have been passed across the placenta. 
The researchers found that cancer cells from the mother and the child had the same mutation in the oncogene BCR-ABL1 — but that this mutation hadn't been inherited, indicating that the cell had passed to the fetus during the pregnancy. 
In general, when the cells did cross the placental barrier, the child's immune system should have recognized them as foreign invaders and destroyed them.  The researchers examined the genes of the cancer cells in the infant and found a deletion mutation. The absence of these molecules on the cancer cells meant that the infant’s immune system would not have recognized that they were foreign.  This study indicates the importance of the immune system to protect patients from leukaemia. ______________________________________________________________________________________ Comments
Certainly, we can't avoid certain risk factors for cancer development. But if we take a good care of our health, an appropriate lifestyle, a suitable physical environment, a healthy diet, we definitely have a lower risk for certain types of cancer development.
Reference:  Mom passes cancer to baby, blogs.nature.com – October 13, 2009  Richard Alleyne, Mother passes on cancer to unborn baby, Telegraph.co.uk, October 13, 2009.  Pregnant women with cancer can pass disease to unborn baby, study shows, The Times, October 13, 2009  Sarah Boseley, Scientists prove cancer can be passed on in the womb, Guardian.co.uk, October 13, 2009